Question 1 of 25
The number is shown with base-ten blocks. If you want to regroup to have exactly ones, what other part of the decomposition must change, and how?
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1st Grade Math
Practice Test 5 for 1st Grade Math: real questions and explanations from the Varsity Tutors practice-test pool.
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Question 1 of 25
The number 74 is shown with base-ten blocks. If you want to regroup to have exactly 14 ones, what other part of the decomposition must change, and how?
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The number 74 is shown with base-ten blocks. If you want to regroup to have exactly 14 ones, what other part of the decomposition must change, and how?
Explanation: To have 14 ones in the decomposition of 74, we need to determine how many tens remain. Since 74 = 60 + 14, we need 6 tens and 14 ones. Choice B gives 64, choice C gives 94, and choice D gives 84 - all incorrect totals.
Review the shapes in the diagram. Jake combines 1 square and 2 quarter-circles to make a shape that looks like a house with a round door. Then he uses his house shape with 1 triangle to create a mountain scene. What shapes are in Jake's final mountain scene?
Explanation: When Jake builds composite shapes, the original shapes remain as parts of the new shape. His final mountain scene contains all the shapes he used: 1 square + 2 quarter-circles (from the house) + 1 triangle = 4 shapes total. Choice B incorrectly treats the house as a single new shape rather than its component parts. Choice C adds extra shapes that weren't used. Choice D incorrectly combines or removes shapes.
At the zoo, Alex sees 5 monkeys playing. Then 3 more monkeys join them. After lunch, Alex sees 2 more monkeys arrive. Some monkeys were sleeping and didn't come out. If there are 15 monkeys total at the zoo, how many monkeys were sleeping?
Explanation: Alex saw 5 + 3 + 2 = 10 monkeys that came out to play. If there are 15 monkeys total, then 15 - 10 = 5 monkeys were sleeping. Choice B (6) would mean only 9 monkeys came out. Choice C (7) would mean only 8 monkeys came out. Choice D (10) would mean no monkeys came out, which contradicts what Alex saw.
Carlos says that 90 is the same as 9 ones. Maya says that 90 is the same as 9 tens. Who is correct?
Explanation: When you see a two-digit number like 90, you need to understand what each digit position means. The digit on the right represents ones, and the digit on the left represents tens. Let's break down the number 90. It has a 9 in the tens place and a 0 in the ones place. This means 90 equals 9 tens plus 0 ones. Since each ten is worth 10 ones, we have 9×10=90. So Maya is absolutely right that 90 is the same as 9 tens. Now let's check each answer choice. Choice A says Carlos is correct just because 90 "has a 9 in it," but this misses the crucial point about place value – where the 9 sits matters more than just its presence. Choice B incorrectly flips the digits, claiming 90 means 0 tens and 9 ones, which would actually give us just 9, not 90. Choice C makes a major error by saying 9 ones equals 9 tens, but 9 ones only equals 9, while 9 tens equals 90 – these are completely different amounts. Choice D correctly identifies that 90 means 9 tens and 0 ones, making Maya right. Remember this strategy: When working with two-digit numbers, always check the place value of each digit. The left digit tells you how many tens, and the right digit tells you how many ones. This will help you avoid confusing which digit represents which amount.
Sofia has 12 balloons and gives away 5; start at 12 and count back 5. How many balloons now?
Explanation: This question tests 1st grade ability to relate counting to addition and subtraction (CCSS.1.OA.5). Counting back is a strategy for subtraction. Start at the first number (minuend) and count backward by the second number (subtrahend). For example, to solve 10−2, start at 10 and count back 2: '9, 8'—the last number you say (8) is the answer. The problem asks to subtract 12−5 by counting back from 12 in a word problem context. Choice C is correct because starting at 12 and counting back 5 gives '11, 10, 9, 8, 7,' so the answer is 7. Choice A is a common error where students count back one too few, perhaps by miscounting the steps; this happens because counting back is more challenging than counting forward for many students. To help students: Demonstrate counting back with number line showing backward movement; use fingers to track counts while saying numbers aloud; practice with physical objects (start with group, remove by counting back); emphasize the last number said is the answer; practice with small subtrahends (1-5) first; connect counting to written subtraction equations.
There are 5 apples in a basket, 5 apples on the table, and 3 apples on the counter. How many apples are there in all?
Explanation: This question tests 1st grade ability to add three whole numbers with sum ≤20 (CCSS.1.OA.2). When adding three numbers, students can use helpful strategies to make it easier. One strategy is to look for two numbers that make 10 (like 6+4 or 7+3), add those first, then add the third number; another strategy is to look for doubles (like 4+4) or numbers that are easy to add together, combine those first, then add the remaining number; students can add in any order because of the commutative and associative properties. The story presents three quantities: 5 apples in a basket, 5 apples on the table, and 3 apples on the counter. Choice C is correct because adding all three numbers gives 5 + 5 + 3 = 13; we can add the doubles 5+5=10 first, then 10+3=13. Choice A is a common error where students only add two of the three numbers, such as 5+5=10, and forget to add the third; this happens because keeping track of three numbers is challenging. To help students: Use physical objects in three groups that students can count and combine; teach making-10 strategy explicitly with pairs that sum to 10; practice doubles facts (3+3, 4+4, 5+5); model different groupings using parentheses: (5+5)+3 or 5+(5+3); use visual representations showing three groups; emphasize that order doesn't matter; have students explain their strategy; practice with various number combinations; connect to real contexts with three groups.
Which makes a rectangle a rectangle?
Explanation: This question tests 1st grade understanding of what defines a rectangle according to CCSS.1.G.1. Defining attributes include four sides, four right-angle corners, opposite sides equal, and being closed. Non-defining attributes such as color or orientation can vary freely. The question asks which attribute makes a rectangle a rectangle, emphasizing structural essentials. Choice B is correct because four sides and four corners capture key defining features. Choice C is a common error where students might think color defines it, as children often remember shapes from colored examples and struggle to abstract beyond visuals. To help students: Color rectangles in various hues while counting sides, discuss 'Purple or blue, still a rectangle if it has four sides!', and use blocks to build and identify rectangles ignoring non-defining traits.
Ben is on step 38 of the stairs. He climbs up 10 more steps. What step is Ben on now?
Explanation: This is an addition problem where you need to find where Ben ends up after climbing more steps. When someone climbs "up" steps, you add to find the new position. Ben starts on step 38 and climbs up 10 more steps. To find his new position, you add: 38+10=48. So Ben is now on step 48. Let's see why the other answers are wrong. Choice A gives 28, which you'd get if you subtracted instead of added (38−10=28). This represents going down steps, not up. Choice B shows 39, which you'd get by adding just 1 instead of 10 (38+1=39). This misreads how many steps Ben climbed. Choice C gives 47, which comes from adding 9 instead of 10 (38+9=47). This is close but uses the wrong number. The correct answer is D) Step 48 because 38+10=48. Remember: when you see "climbs up," "goes up," or "moves forward," you add. When you see "goes down," "steps back," or "climbs down," you subtract. Also, double-check that you're using the exact numbers from the problem—it's easy to misread 10 as 1 or mix up the starting position.
If 8+3=11, what is 3+8?
Explanation: This question tests 1st grade understanding of properties of operations as addition strategies (CCSS.1.OA.3). The commutative property of addition means that the order of numbers doesn't matter when adding: a + b = b + a. For example, if we know that 8 + 3 = 11, we also know that 3 + 8 = 11 without recalculating, which helps in choosing an easier order like starting with the larger number. The problem gives 8 + 3 = 11 and asks for the value of 3 + 8, noting that order doesn’t change the sum. Choice B is correct because the commutative property tells us 3 + 8 gives the same sum as 8 + 3, which is 11. Choice A is a common error where students might add incorrectly or think reversing order increases the sum, often because properties are abstract and need concrete examples to understand. To help students: Provide many concrete examples showing both orders give the same answer; use physical objects to demonstrate the commutative property (count group A then B, or B then A—same total); practice with equations side by side (8+3=11, 3+8=11); use visual models like ten-frames; emphasize 'order doesn’t matter' for commutative; connect properties to efficient mental math strategies.
Chen has 45 points and loses 10; how many points now?
Explanation: This question tests 1st grade ability to mentally find 10 more or 10 less than a two-digit number (CCSS.1.NBT.C.5). To add or subtract 10 mentally, use place value understanding: since 10 equals 1 ten and 0 ones, adding 10 means adding 1 ten (so the tens digit increases by 1), while the ones digit stays the same. For example, 34 + 10 = 44 because 3 tens + 1 ten = 4 tens, and the 4 ones remain unchanged; similarly, subtracting 10 means removing 1 ten (tens digit decreases by 1), while ones stay the same: 67 - 10 = 57 because 6 tens - 1 ten = 5 tens, and the 7 ones remain unchanged. The question presents a word problem where Chen starts with 45 points and subtracts 10. Choice C is correct because subtracting 10 from 45 means removing 1 ten: 4 tens - 1 ten = 3 tens, ones stay 5, giving 35. Choice A is a common error where students reverse the operation and add 10 instead, resulting in 55; this happens because students sometimes confuse addition and subtraction directions. To help students: Use base-10 blocks to show physically removing 1 ten-rod while ones stay constant; practice on hundred charts (subtract 10 = up one row); emphasize pattern 'tens change by 1, ones stay same'; provide many examples showing ones digit constant; connect to skip counting by 10s; use number lines with jumps of 10; practice mental math daily with quick '10 less' questions; explain why this is efficient compared to counting by ones; make the place value connection explicit.
Sofia has 6 tens and 8 ones. Add 1 ten. What number?
Explanation: This question tests 1st grade ability to mentally find 10 more or 10 less than a two-digit number (CCSS.1.NBT.5). To add or subtract 10 mentally, use place value understanding: since 10 equals 1 ten and 0 ones, adding 10 means adding 1 ten (so the tens digit increases by 1), while the ones digit stays the same. For example, 34 + 10 = 44 because 3 tens + 1 ten = 4 tens, and the 4 ones remain unchanged. Similarly, subtracting 10 means removing 1 ten (tens digit decreases by 1), while ones stay the same: 67 - 10 = 57 because 6 tens - 1 ten = 5 tens, and the 7 ones remain unchanged. The stimulus describes Sofia with 6 tens and 8 ones, adding 1 ten, asking what number. Choice B is correct because adding 1 ten to 6 tens and 8 ones gives 7 tens and 8 ones, which is 78. Choice D is a common error where students don't change the tens digit and keep it as 68; this happens because understanding 10 as 1 ten is abstract and the place value connection isn't automatic yet. To help students: Use base-10 blocks to show physically adding/removing 1 ten-rod while ones stay constant; practice on hundred charts (add 10 = down one row, subtract 10 = up one row); emphasize pattern 'tens change by 1, ones stay same'; provide many examples showing ones digit constant; connect to skip counting by 10s; use number lines with jumps of 10; practice mental math daily with quick '10 more/less' questions; explain why this is efficient compared to counting by ones; make the place value connection explicit.
Kevin had 11 toy blocks. He used 6 blocks to build a tower. Later, his sister gave him 3 more blocks. Then he used 2 more blocks for his tower. Which number sentence shows how many blocks Kevin had after building the first part of his tower but before his sister gave him more blocks?
Explanation: When you see a story problem with multiple steps, you need to carefully identify what the question is asking for. This problem asks specifically for how many blocks Kevin had "after building the first part of his tower but before his sister gave him more blocks." Let's follow Kevin's story step by step. He started with 11 blocks and used 6 blocks to build a tower. At this point, before anything else happened, he had 11−6=5 blocks left. This is exactly what the question asks for, making D the correct answer. Now let's see why the other choices are wrong. Choice A (11−6−2=3) includes the 2 blocks he used later, but the question asks for the number before that happened. Choice B (11−6+3=8) adds the 3 blocks his sister gave him, but again, the question asks for the amount before she gave him any blocks. Choice C (5+3−2=6) starts with the correct number (5) but then continues the story by adding sister's blocks and subtracting the final 2 blocks used. The key strategy here is to read the question carefully and stop your calculation at the exact moment it asks about. Don't get distracted by extra information in the story that happens after the point you're looking for. Circle or underline the specific time period the question asks about to help you focus.
Look at the number line. Amy started at 16 and moved backward 6 ones. Where did she end up?
Explanation: Starting at 16 (1 ten and 6 ones) and moving backward 6 ones: 16 - 6 = 10, which is 1 ten and 0 ones. Choice B shows moving forward instead of backward. Choice C shows subtracting only 4 instead of 6. Choice D shows subtracting only 2 instead of 6.
Count by 10s: 80, 90, 100, . What comes next?
Explanation: This question tests 1st grade ability to count to 120, starting at any number less than 120 (CCSS.1.NBT.1). Students must know the counting sequence from 1 to 120 and be able to start counting at any number within that range. Key skills include recognizing what number comes next, reading numerals accurately, writing numerals correctly, and representing quantities with written numerals. Important transitions occur at decade boundaries (29→30, 99→100) and in teen numbers (11-19). The stimulus shows a skip counting sequence by 10s: 80, 90, 100, . Choice B is correct because 110 comes next when counting by tens after 100. Choice D is a common error where students might jump to 120, skipping 110; this happens because skip counting requires understanding patterns in tens. To help students: Use hundreds charts for visual reference; practice counting aloud daily, starting from different numbers; emphasize decade transitions explicitly (29→30, 99→100); provide practice reading and writing numerals with immediate feedback; use physical objects organized in groups of 10 to show quantities; teach teen numbers as 'ten and ' (thirteen = 10 and 3); practice skip counting by 10s regularly; connect counting to real contexts (pages, calendar dates, collections).
Sam puts some red blocks and some blue blocks in a box to make 10 blocks total. He uses 4 red blocks. Then he takes out 1 red block. How many blue blocks does he need now to make 10 blocks total?
Explanation: When you see a word problem with multiple steps, it's important to work through each action in order to keep track of what's happening. Let's follow what Sam does step by step. First, he puts 4 red blocks in the box. Then he takes out 1 red block, leaving him with 4−1=3 red blocks in the box. Now he needs enough blue blocks to make 10 blocks total. Since he has 3 red blocks, he needs 10−3=7 blue blocks to reach his goal of 10 blocks total. Let's check why each answer choice is wrong or right. Choice A) says 6 blue blocks are needed, but 3+6=9 blocks total, which is one short of the target. Choice C) suggests 5 blue blocks, giving us 3+5=8 blocks total, which is two short. Choice D) says 4 blue blocks, resulting in 3+4=7 blocks total, which is three short of the goal. Choice B) correctly identifies that 7 blue blocks are needed because 3+7=10 blocks total. When solving multi-step word problems, slow down and track each change as it happens. Don't just focus on the starting numbers—pay attention to how the situation changes throughout the problem. Drawing a simple picture or writing down each step can help you avoid mixing up the original amounts with the final amounts.
Maya has 7 stickers. She wants to have 10 stickers total. Her friend gives her some stickers, and her sister gives her some stickers too. Which shows a way Maya could get exactly 10 stickers?
Explanation: When you see a problem asking "how many more" to reach a target number, you need to find the missing amount and then check if it can be split correctly between different sources. Maya starts with 7 stickers and wants 10 total. First, figure out how many more she needs: 10−7=3 stickers. Now you need to find which answer choice shows two amounts that add up to exactly 3. Let's check each option by adding what the friend and sister give: Choice A gives us 2+1=3 stickers total. This matches exactly what Maya needs! With her original 7 stickers plus these 3 new ones, she'll have 7+3=10 stickers. Choice B shows 3+2=5 stickers, which is too many. Maya would end up with 7+5=12 stickers, going over her goal. Choice C gives 4+1=5 stickers, also too many for the same reason as choice B. Choice D provides 1+4=5 stickers, again resulting in 12 total stickers instead of 10. The key strategy here is to always find the "gap" first (how many more you need), then check which combination of parts adds up to fill that exact gap. Don't get distracted by the different sources - focus on whether the total amount matches what's missing.
Sarah has 8 crayons in her box. She lends some crayons to her friend Tom. After lending them, she has 5 crayons left. Then her teacher gives her 6 new crayons. How many crayons does Sarah have now?
Explanation: This is a word problem that involves addition and subtraction in sequence. When you see a story problem with multiple steps, you need to work through each event in the order it happens. Let's follow what happens to Sarah's crayons step by step. Sarah starts with 8 crayons. When she lends some to Tom, she has 5 left. Then her teacher gives her 6 more crayons. To find how many she has now, you add the crayons she had left (5) to the new crayons from her teacher (6): 5+6=11 crayons. Looking at the wrong answers, choice A (21) likely comes from adding all the numbers in the problem: 8+5+6+2=21, but this doesn't make sense because you can't just add every number you see. Choice B (14) might come from adding the original amount to the teacher's gift: 8+6=14, but this ignores that Sarah lent some crayons away first. Choice C (19) could result from adding 8+5+6=19, which incorrectly treats the 5 remaining crayons as additional crayons rather than what's left from the original 8. When solving multi-step word problems, always track what happens in order. Don't just add all the numbers you see—think about what each number represents in the story and how the events connect to each other.
Sofia has 9 crayons. Some are red. Now she has 15 crayons. How many crayons did Sofia get? 9+?=15
Explanation: This question tests 1st grade ability to solve addition and subtraction word problems within 20 (CCSS.1.OA.1). This is an adding to problem with change unknown. We start with one amount, end with another, and find what was added. The story tells us Sofia has 9 crayons and now has 15 crayons after getting some more. Choice B is correct because to find how many crayons Sofia got, we subtract: 15−9=6, or add up: 9+6=15. We can represent this as 9+?=15. Choice D is a common error where students calculate incorrectly, such as subtracting smaller from larger in the wrong order like 9−4=5, but actually it's 15−9 or similar miscount. This happens because in change unknown problems, students don't recognize they need to work backwards. To help students: Act out problems with physical objects; draw pictures representing the situation; teach problem type structures explicitly (adding to, taking from, putting together, taking apart, comparing); use consistent language for each type; practice writing equations with boxes for unknowns; emphasize keywords carefully ('more' can mean add or compare); model thinking aloud ('I started with 9, ended with 15, need to find what was added, so I subtract or add up'); practice all unknown positions; connect to familiar experiences.
Look at the sequence: 18, 19, , 21. What number is missing?
Explanation: This question tests 1st grade ability to count to 120, starting at any number less than 120 (CCSS.1.NBT.1). Students must know the counting sequence from 1 to 120 and be able to start counting at any number within that range. Key skills include recognizing what number comes next, reading numerals accurately, writing numerals correctly, and representing quantities with written numerals. Important transitions occur at decade boundaries (29→30, 99→100) and in teen numbers (11-19). The stimulus shows a number sequence with a missing number: 18, 19, , 21. Choice A is correct because 20 fills the gap in the sequence 18, 19, , 21, marking a decade transition. Choice B is a common error where students confuse 10 with 20, mixing up teen and decade numbers; this happens because teen numbers have unusual names that don't match their structure. To help students: Use hundreds charts for visual reference; practice counting aloud daily, starting from different numbers; emphasize decade transitions explicitly (29→30, 99→100); provide practice reading and writing numerals with immediate feedback; use physical objects organized in groups of 10 to show quantities; teach teen numbers as 'ten and ' (thirteen = 10 and 3); practice skip counting by 10s regularly; connect counting to real contexts (pages, calendar dates, collections).
Amir has 28 blocks. He gets 10 more. How many blocks?
Explanation: This question tests 1st grade ability to mentally find 10 more or 10 less than a two-digit number (CCSS.1.NBT.5). To add or subtract 10 mentally, use place value understanding: since 10 equals 1 ten and 0 ones, adding 10 means adding 1 ten (so the tens digit increases by 1), while the ones digit stays the same. For example, 34+10=44 because 3 tens+1 ten=4 tens, and the 4 ones remain unchanged. Similarly, subtracting 10 means removing 1 ten (tens digit decreases by 1), while ones stay the same: 67−10=57 because 6 tens−1 ten=5 tens, and the 7 ones remain unchanged. The stimulus describes Amir with 28 blocks getting 10 more, asking how many blocks. Choice A is correct because adding 10 to 28 means adding 1 ten: 2 tens+1 ten=3 tens, ones stay 8, giving 38. Choice B is a common error where students add 1 instead of 10, changing the ones digit to 9; this happens because understanding 10 as 1 ten is abstract and students sometimes focus on the digit '10' rather than its place value meaning. To help students: Use base-10 blocks to show physically adding/removing 1 ten-rod while ones stay constant; practice on hundred charts (add 10 = down one row, subtract 10 = up one row); emphasize pattern 'tens change by 1, ones stay same'; provide many examples showing ones digit constant; connect to skip counting by 10s; use number lines with jumps of 10; practice mental math daily with quick '10 more/less' questions; explain why this is efficient compared to counting by ones; make the place value connection explicit.
To add 7+5, start at 7 and count on 5: 8, 9, 10, 11, 12. What is the sum?
Explanation: This question tests 1st grade ability to relate counting to addition and subtraction (CCSS.1.OA.5). Counting on is an efficient strategy for addition. Instead of counting from 1, start at one of the addends and count forward by the other addend. For example, to solve 7 + 5, start at 7 and count forward 5 numbers: '8, 9, 10, 11, 12'—the last number you say (12) is the answer. The problem asks to add 7 + 5 by counting on from 7. Choice B is correct because starting at 7 and counting on 5 gives '8, 9, 10, 11, 12,' so the answer is 12. Choice A is a common error where students count one extra, perhaps starting from 7 as the first count; this happens because distinguishing between the starting point and counts requires practice. To help students: Model counting on with number lines showing clear starting point and forward jumps; use fingers to track counts while saying numbers aloud; emphasize NOT starting at 1 for counting on; practice with physical objects (start with group, add more by counting on); compare efficiency of counting on vs counting all from 1; practice with small addends (1-5) first; connect counting to written addition equations.
Keisha has 6 points. She scores 7 more points. How many points does Keisha have now?
Explanation: This question tests 1st grade ability to solve addition and subtraction word problems within 20 (CCSS.1.OA.1). This is an adding to problem with result unknown. We start with one amount and add more to find the total. The story tells us Keisha has 6 points and scores 7 more. Choice B is correct because to find how many points Keisha have now, we add: 6+7=13. We can represent this as an equation with unknown: 6+7=?. Choice A is a common error where students might add incorrectly, like 6+6=12, miscounting. To help students: Act out problems with physical objects; draw pictures representing the situation; teach problem type structures explicitly (adding to, taking from, putting together, taking apart, comparing); use consistent language for each type; practice writing equations with boxes for unknowns; emphasize keywords carefully ('more' can mean add or compare); model thinking aloud ('I started with 6, got 7 more, now I need to find the total, so I add'); practice all unknown positions; connect to familiar experiences.
Jamal turns a square sideways. Is it still a square?
Explanation: This question tests 1st grade understanding that orientation (position or rotation) is a non-defining attribute of shapes (CCSS.1.G.1). A shape remains the same shape regardless of how it's turned, flipped, or positioned on the page. The question asks if a square turned sideways is still a square. Choice A (Yes, it is still a square) is correct because rotating a shape doesn't change its defining attributes—it still has 4 equal sides and 4 right corners. Choices B and C incorrectly suggest the shape changes type, while Choice D introduces an irrelevant color condition. Young students often think a rotated square becomes a 'diamond' because they focus on orientation rather than attributes. To help students: Physically rotate shape cutouts to show they don't change; trace shapes in different orientations; emphasize 'A square is always a square, even when it's turned around!'
Sofia has 16 straws: 1 bundle and some loose. How many loose ones?
Explanation: This question tests 1st grade understanding that teen numbers (11-19) are composed of 1 ten and ones (CCSS.1.NBT.2.b). All teen numbers—11, 12, 13, 14, 15, 16, 17, 18, and 19—have the same structure: exactly 1 ten plus some ones (between 1 and 9 ones). For example, 16 is made of 1 ten and 6 ones, which we can also write as 10 + 6 = 16, building place value knowledge. The scenario describes Sofia with 16 straws as 1 bundle (ten) and some loose ones. Choice B is correct because 16 is composed of exactly 1 ten and 6 ones. Choice C is a common error where students think the total is all ones without recognizing the bundle as a ten, often because teen numbers' structure isn't intuitive from their names alone. To help students: Use base-10 blocks extensively—always show 1 ten-rod plus unit cubes for teens; connect number names to structure ('six-teen' = 6 + 10); practice building each teen number with 1 ten and ones repeatedly; emphasize ALL teen numbers have 1 ten, only the ones vary; use ten-frames with one full frame (the ten) and partial second frame (the ones); write equations (10 + 6 = 16); practice decomposing teens into 10 + ones; compare teens to show same ten structure.
Sofia shows 1 ten-rod and 5 cubes. 1 ten and 5 ones makes .
Explanation: This question tests 1st grade understanding that teen numbers (11-19) are composed of 1 ten and ones (CCSS.1.NBT.2.b). All teen numbers—11, 12, 13, 14, 15, 16, 17, 18, and 19—have the same structure: exactly 1 ten plus some ones (between 1 and 9 ones). For example, 15 is made of 1 ten and 5 ones, which we can also write as 10 + 5 = 15. This structure helps students understand place value and connects the teen number names to their composition. The stimulus shows Sofia with 1 ten-rod and 5 cubes, representing 1 ten and 5 ones making a teen number. Choice A is correct because 1 ten plus 5 ones equals 15. Choice C is a common error where students reverse the digits (51 instead of 15). To help students: Use base-10 blocks extensively—always show 1 ten-rod plus unit cubes for teens; connect number names to structure ('fif-teen' = 5 + 10); practice building each teen number with 1 ten and ones repeatedly; emphasize ALL teen numbers have 1 ten, only the ones vary; use ten-frames with one full frame (the ten) and partial second frame (the ones); write equations (10 + 5 = 15); practice decomposing teens into 10 + ones; compare teens to show same ten structure.