Adding & Subtracting 10 - 1st Grade Math

1. Maya is at 45 on a number line. What is 10 less?
   Answer: 35 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 involves Maya at 45 on a number line, asking for 10 less. Choice B 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 subtract 1 instead of 10, changing the ones digit to 4; 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.
2. Tom thinks of a number. When he adds $10$ to his number, he gets $65$. What number did Tom think of?
   Answer: $55$ Since Tom's number plus 10 equals 65, his number must be 65 minus 10, which is 55. 75 mistakenly adds 10 to 65 instead of subtracting. 64 subtracts only 1 instead of 10. 66 adds 1 instead of subtracting 10. Only 55 correctly undoes the plus 10 to find Tom's starting number.
3. Sofia has 38 cards and gives away 10; how many left?
   Answer: 28 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 Sofia starts with 38 cards and subtracts 10. Choice A is correct because subtracting 10 from 38 means removing 1 ten: 3 tens - 1 ten = 2 tens, ones stay 8, giving 28. Choice B is a common error where students subtract 1 instead of 10, resulting in 37; this happens because understanding 10 as 1 ten is abstract and students sometimes change the ones digit. 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.
4. Look at the number line. If you start at $34$ and move to find $10$ more, where do you end up?
   Answer: $44$ Starting at 34 and moving 10 more means adding 10, which lands on 44. 35 only moves 1 space, not 10. 24 moves 10 spaces in the wrong direction (subtracting instead of adding). 33 moves only 1 space in the wrong direction. Only 44 correctly adds 10 to 34.
5. Find 10 less than 82; tens change by 1, ones stay.
   Answer: 72 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 asks to find 10 less than 82, noting tens change by 1 while ones stay the same. Choice A is correct because subtracting 10 from 82 means removing 1 ten: 8 tens - 1 ten = 7 tens, ones stay 2, giving 72. Choice B is a common error where students subtract 1 from the ones digit instead, resulting in 81; this happens because students sometimes focus on the digit '10' rather than its place value meaning. 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.
6. Maya has $23$ stickers. She gives away $10$ stickers to her friend, then finds $10$ more stickers in her backpack. How many stickers does Maya have now?
   Answer: $23$ stickers because she has the same amount Maya starts with 23 stickers. After giving away 10, she has 23 - 10 = 13. Then she finds 10 more: 13 + 10 = 23. She ends up with the same amount she started with. Choice A shows only the first step. Choice C incorrectly adds 10 to the starting amount. Choice D incorrectly adds 20 to the starting amount.
7. What is $45-10$?
   Answer: 35 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, subtracting 10 means removing 1 ten (tens digit decreases by 1), while the ones digit stays the same. For example, 45 - 10 = 35 because 4 tens - 1 ten = 3 tens, and the 5 ones remain unchanged. The scenario involves finding 45 - 10 mentally, with a hint that only the tens digit changes. 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 add 10 instead of subtracting, resulting in 55; this happens because they reverse the operation, perhaps misreading the subtraction sign. 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.
8. Amir has 23 toy cars and gets 10 more. How many toy cars does Amir have now?
   Answer: 33 Amir starts with 23 toy cars and gets 10 more, so 23 plus 10 equals 33. 24 adds only 1 instead of 10. 43 adds 20 instead of 10. 23 repeats his starting amount without adding the new cars. Only 33 correctly adds Amir's starting cars and the new ones he received.
9. Find $56+10$ mentally. Change the tens digit, not the ones.
   Answer: 66 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 asks to find 56 + 10 mentally by changing the tens digit, not the ones. Choice C is correct because adding 10 to 56 means adding 1 ten: 5 tens + 1 ten = 6 tens, ones stay 6, giving 66. Choice A is a common error where students add 1 instead of 10, changing the ones digit to 7; 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.
10. Keisha has 93 points. She loses 10 points. How many points?
    Answer: 83 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 Keisha with 93 points losing 10, asking how many points. Choice A is correct because subtracting 10 from 93 means removing 1 ten: 9 tens - 1 ten = 8 tens, ones stay 3, giving 83. Choice B is a common error where students subtract 9 instead of 10 or change the ones digit to 4; this happens because counting by ones is a fallback that can lead to errors, especially near decade numbers. 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.
11. Sofia jumps 10 forward on a number line from 68. Where does she land?
    Answer: 78 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, 68 + 10 = 78 because 6 tens + 1 ten = 7 tens, and the 8 ones remain unchanged. The scenario involves Sofia jumping 10 forward on a number line from 68. Choice A is correct because jumping 10 forward from 68 means adding 1 ten: 6 tens + 1 ten = 7 tens, ones stay 8, giving 78. Choice B is a common error where students add 1 instead of 10, resulting in 69; this happens because understanding 10 as 1 ten is abstract and they might count by ones instead. To help students: Use base-10 blocks to show physically adding 1 ten-rod while ones stay constant; practice on hundred charts (add 10 = down 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' questions; explain why this is efficient compared to counting by ones; make the place value connection explicit.
12. Chen has $67$ stickers. He gives away $10$. How many stickers does he have now?
    Answer: 57 Chen starts with 67 stickers and gives away 10, so 67 minus 10 equals 57. 56 subtracts one too many. 66 subtracts only 1 instead of 10. 77 adds instead of subtracting. Only 57 correctly shows how many stickers Chen has left.
13. Sarah has $58$ marbles. She wants to have $10$ fewer marbles. How many marbles should she give away?
    Answer: $10$ marbles, which is exactly $10$ fewer Sarah should give away 10 marbles to end up with exactly 10 fewer than she started with. Giving away 9 marbles is one short of the amount needed. 48 is how many marbles Sarah would have left, not how many she should give away. Giving away 11 marbles is one more than needed. Only giving away 10 marbles results in exactly 10 fewer marbles.
14. Jake says $10$ less than $53$ is $52$. Emma says $10$ less than $53$ is $43$. Who is correct and why?
    Answer: Emma is correct because $53 - 10 = 43$ To find 10 less than 53, subtract 10: 53 - 10 = 43. Emma is correct. Jake subtracted 1 instead of 10, which gives 1 less, not 10 less. Choice A explains Jake's error but says he's correct. Choice C gives 10 more instead of 10 less. Choice D is incorrect because only one answer can be right.
15. Lisa has $32$ books. She gets $10$ more books for her birthday, then gives $20$ books to the library. How many books does she have left?
    Answer: $22$ books after all the changes This is a multi-step word problem that requires you to track changes to Lisa's collection of books. When you see problems with multiple operations, work through them step by step in the order they happen. Start with Lisa's original amount: $32$ books. First, she gets $10$ more books for her birthday, so you add: $32 + 10 = 42$ books. Next, she gives $20$ books to the library, so you subtract: $42 - 20 = 22$ books. Lisa ends up with $22$ books after all the changes. Looking at the wrong answers: Choice B ($12$ books) happens if you subtract both the birthday books and library books from the original amount: $32 - 10 - 20 = 2$, or if you make an error in your final subtraction. Choice C ($42$ books) is the amount Lisa had after getting birthday books but before giving any to the library—this happens when you forget the final step. Choice D ($62$ books) occurs if you add all the numbers together instead of following the story: $32 + 10 + 20 = 62$. The correct answer is A. For multi-step word problems, always follow the story in order and keep track of your running total after each operation. Circle or underline key words like "gets" (addition) and "gives away" (subtraction) to help you identify which operation to use at each step.
16. Which number becomes $71$ when you add $10$ to it?
    Answer: $61$ because $61 + 10 = 71$ When you see a question asking "which number becomes [something] when you add [amount] to it," you're looking for the starting number in an addition problem. Think of it like working backwards from the answer. The question asks which number becomes $71$ when you add $10$ to it. This means you need to find what number plus $10$ equals $71$. You can think of it as: ? + $10$ = $71$. To solve this, you can count backwards from $71$ by $10$, or think about what $71 - 10$ equals. Since $71 - 10 = 61$, the starting number must be $61$. You can check this: $61 + 10 = 71$ ✓ Looking at the choices: Answer A is correct because $61 + 10 = 71$, which matches exactly what the question asks for. Answer B is wrong because $60 + 10 = 70$, not $71$ - this gives you a number that's one less than what you need. Answer C gets the direction mixed up - it shows $71 + 10 = 81$, but the question asks what number becomes $71$ when you add $10$, not what $71$ becomes when you add $10$. Answer D uses the wrong operation - it shows $62 + 9 = 71$, but the question specifically asks about adding $10$, not $9$. Remember: When a question asks "which number becomes X when you add Y," work backwards by subtracting Y from X to find your starting number, then double-check your answer by adding forward.
17. Find 10 more than 47 without counting by ones.
    Answer: 57 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 asks to find 10 more than 47 without counting by ones. Choice B is correct because adding 10 to 47 means adding 1 ten: 4 tens + 1 ten = 5 tens, ones stay 7, giving 57. Choice A is a common error where students add 1 instead of 10, changing the ones digit to 8; this happens because counting by ones is a fallback that can lead to errors or students 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.
18. Which number is $10$ less than $47$?
    Answer: $37$ Ten less than 47 is 37, found by subtracting 10. 46 is only one less than 47, not ten less. 36 subtracts one too many, going past ten. 57 adds 10 instead of subtracting it. Only 37 correctly shows 47 minus 10.
19. Without counting by ones, what is $56+10$?
    Answer: 66 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 asks to find 56 + 10 without counting by ones, emphasizing mental strategy. Choice A is correct because adding 10 to 56 means adding 1 ten: 5 tens + 1 ten = 6 tens, ones stay 6, giving 66. Choice D is a common error where students do nothing or forget to change the tens digit, leaving it as 56; this happens because counting by ones is a fallback that can lead to errors when not using the efficient mental strategy. To help students: Use base-10 blocks to show physically adding 1 ten-rod while ones stay constant; practice on hundred charts (add 10 = down 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' questions; explain why this is efficient compared to counting by ones; make the place value connection explicit.
20. Chen has 45 points and loses 10; how many points now?
    Answer: 35 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.

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