1st Grade Math Quiz: Subtraction As An Unknown Addend Problem
20 questions · exam conditions
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Subtraction As An Unknown Addend ProblemQuestion 1 of 20

Maya has 6 stickers. How many more stickers does she need to have 14?

8
6
9
20
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1st Grade Math Quiz

1st Grade Math Quiz: Subtraction As An Unknown Addend Problem

Practice Subtraction As An Unknown Addend Problem in 1st Grade Math with focused quiz questions that help you check what you know, review explanations, and build confidence with test-style prompts.

What this quiz covers

This quiz focuses on Subtraction As An Unknown Addend Problem, giving you a quick way to practice the rules, question types, and explanations that matter most for 1st Grade Math.

How to use this quiz

Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

All questions

Question 1

Maya has 6 stickers. How many more stickers does she need to have 14?

  1. 8 (correct answer)
  2. 6
  3. 9
  4. 20
Explanation: Maya needs 14 minus 6 more stickers, which is 8. 6 repeats her starting amount instead of finding how many more she needs. 9 is one more than the correct amount. 20 adds the two numbers together instead of finding the difference. Only 8 correctly shows how many more stickers Maya needs.

Question 2

Find 17−1217-12 by thinking 12+?=1712+?=17. What number is ???

  1. 4
  2. 12
  3. 5 (correct answer)
  4. 29
Explanation: This question tests 1st grade understanding of subtraction as an unknown-addend problem (CCSS.1.OA.4). Subtraction can be solved by thinking about addition. Instead of 'taking away,' we can ask 'what do I add to get to the total?' For example, to solve 10 - 8, we can think '8 plus what equals 10?' or write it as 8 + ? = 10. Finding the missing addend (2) gives us the answer to the subtraction problem. This strategy is often easier than counting back, especially when the numbers are close together. The problem asks to find 17-12 by thinking of it as the unknown addend equation 12 + ? = 17, asking for the missing number. Choice C is correct because when we add 5 to 12, we get 17, so 12 + 5 = 17, which means 17 - 12 = 5. Choice D is a common error where students add the two numbers instead of finding the difference, getting 29, because the connection between addition and subtraction is abstract. To help students: Use part-part-whole diagrams showing the relationship; practice on number lines with counting-on (start at smaller number, count forward to larger); teach fact families explicitly (12+5=17, 5+12=17, 17-12=5, 17-5=12); use 'think addition' language consistently ('to subtract 17-12, think what plus 12 equals 17'); provide many examples connecting subtraction to unknown addend; emphasize that this strategy is especially useful when numbers are close together; use context problems where 'how many more needed' feels natural; practice writing both forms of the problem.

Question 3

9−4=?9-4=? matches 4+?=94+?=9. What is the missing number?

  1. 4
  2. 13
  3. 5 (correct answer)
  4. 6
Explanation: This question tests 1st grade understanding of subtraction as an unknown-addend problem (CCSS.1.OA.4). Subtraction can be solved by thinking about addition, where instead of taking away, we ask what number added to the subtrahend equals the minuend. For example, to solve 9 - 4, we can think '4 plus what equals 9?' or write it as 4 + ? = 9, and finding the missing addend (5) gives the subtraction answer. This strategy highlights the inverse relationship between addition and subtraction, making it easier for young learners to compute differences by counting on. The problem presents the subtraction 9 - 4 as matching the unknown addend equation 4 + ? = 9. Choice C is correct because when we add 5 to 4, we get 9, so 4 + 5 = 9, which means 9 - 4 = 5. Choice B is a common error where students add the two numbers instead of finding the difference, resulting in 13, which happens because the connection between addition and subtraction can be abstract for beginners. To help students: Use part-part-whole diagrams showing the relationship; practice on number lines with counting-on (start at 4, count forward to 9); teach fact families explicitly (4+5=9, 5+4=9, 9-4=5, 9-5=4); use 'think addition' language consistently ('to subtract 9-4, think what plus 4 equals 9'); provide many examples connecting subtraction to unknown addend; emphasize that this strategy is especially useful when numbers are close together; use context problems where 'how many more needed' feels natural; practice writing both forms of the problem.

Question 4

Emma has 1212 marbles in a bag. She takes out some marbles and gives them to her brother. Now she has 77 marbles left in the bag. Emma wants to use the "unknown-addend" way to figure out how many marbles she gave away. What should Emma think?

  1. "1212 plus 77 equals what number?" Then subtract to find what she gave away.
  2. "1212 plus what number equals 77?" Then the answer is how many she gave away.
  3. "77 plus what number equals 1212?" Then the answer is how many she gave away. (correct answer)
  4. "What number plus 1212 equals 77?" Then add to find what she gave away.
Explanation: When you have a subtraction story problem, you can solve it using the "unknown-addend" method by thinking of it as an addition problem with a missing number. Emma started with 1212 marbles, gave some away, and has 77 left. You need to find how many she gave away. The key is to set up the addition equation correctly. Think about what you know: Emma has 77 marbles left, and if you add back the marbles she gave away, you'll get her original 1212 marbles. This means: 77 plus the unknown number of marbles she gave away equals 1212. So Emma should think "77 plus what number equals 1212?" The answer to that question is 55, which is how many marbles she gave away. Let's see why the other choices don't work. Choice A suggests adding 1212 plus 77 first, but this gives you 1919, which doesn't help solve the problem directly. Choice B asks "1212 plus what equals 77?" but this doesn't make sense because you can't add a positive number to 1212 and get a smaller number like 77. Choice D asks "what plus 1212 equals 77?" which would give you a negative number, and you can't give away negative marbles. Remember this pattern: when solving "take away" problems with unknown addends, start with what's left, add the unknown amount, and set it equal to what you started with. This connects subtraction to addition in a way that's easier to visualize.

Question 5

Find 15−815-8 by solving 8+?=158+?=15. What is ???

  1. 7 (correct answer)
  2. 8
  3. 6
  4. 23
Explanation: This question tests 1st grade understanding of subtraction as an unknown-addend problem (CCSS.1.OA.4). Subtraction can be solved by thinking about addition, where instead of 'taking away,' we ask 'what do I add to get to the total?' For example, to solve 10 - 3, we can think '3 plus what equals 10?' or write it as 3 + ? = 10, and finding the missing addend (7) gives the subtraction answer. This strategy leverages the inverse relationship between addition and subtraction, making it easier for young learners to use known addition facts. The problem asks to find 15 - 8 by solving the unknown addend equation 8 + ? = 15. Choice A (7) is correct because when we add 7 to 8, we get 15, so 8 + 7 = 15, which means 15 - 8 = 7. Choice B (8) is a common error where students use one of the numbers from the problem, such as repeating the 8, which happens because they may not fully understand that subtraction and finding the unknown addend are equivalent. To help students: Use part-part-whole diagrams showing the relationship; practice on number lines with counting-on (start at smaller number, count forward to larger); teach fact families explicitly (8+7=15, 7+8=15, 15-8=7, 15-7=8); use 'think addition' language consistently ('to subtract 15-8, think what plus 8 equals 15'); provide many examples connecting subtraction to unknown addend; emphasize that this strategy is especially useful when numbers are close together; use context problems where 'how many more needed' feels natural; practice writing both forms of the problem.

Question 6

Jamal counts on: 11−411-4 means 4+?=114+?=11. What number is ???

  1. 7 (correct answer)
  2. 6
  3. 4
  4. 15
Explanation: Since 4+7=114+7=11, the missing number is 7. 6 is one less than the correct number. 4 repeats a number already in the problem instead of finding the missing one. 15 adds 4 and 11 together instead of finding the difference. Only 7 correctly completes 4+?=114+?=11.

Question 7

Find 10−710-7 by thinking 7+?=107+?=10. What is ???

  1. 2
  2. 3 (correct answer)
  3. 17
  4. 7
Explanation: This question tests 1st grade understanding of subtraction as an unknown-addend problem (CCSS.1.OA.4). Subtraction can be solved by thinking about addition. Instead of 'taking away,' we can ask 'what do I add to get to the total?' For example, to solve 10 - 8, we can think '8 plus what equals 10?' or write it as 8 + ? = 10. Finding the missing addend (2) gives us the answer to the subtraction problem. This strategy is often easier than counting back, especially when the numbers are close together. The problem asks to find 10-7 by thinking of it as the unknown addend equation 7 + ? = 10, asking for the missing number. Choice B is correct because when we add 3 to 7, we get 10, so 7 + 3 = 10, which means 10 - 7 = 3. Choice C is a common error where students add the two numbers instead of finding the difference, getting 17, because students may be more familiar with the 'taking away' model of subtraction. To help students: Use part-part-whole diagrams showing the relationship; practice on number lines with counting-on (start at smaller number, count forward to larger); teach fact families explicitly (7+3=10, 3+7=10, 10-7=3, 10-3=7); use 'think addition' language consistently ('to subtract 10-7, think what plus 7 equals 10'); provide many examples connecting subtraction to unknown addend; emphasize that this strategy is especially useful when numbers are close together; use context problems where 'how many more needed' feels natural; practice writing both forms of the problem.

Question 8

Jamal has 9 toy cars. How many more to have 14? (9+?=149+?=14)

  1. 4
  2. 23
  3. 5 (correct answer)
  4. 9
Explanation: This question tests 1st grade understanding of subtraction as an unknown-addend problem (CCSS.1.OA.4). Subtraction can be solved by thinking about addition, where instead of taking away, we ask what number added to the known part equals the whole. For example, if someone has 9 and wants 14, we can think '9 plus what equals 14?' or write it as 9+?=149 + ? = 14, and finding the missing addend (5) solves how many more are needed. This strategy highlights the inverse relationship between addition and subtraction, making it easier for young learners to compute differences by counting on. The problem presents a real-world context of having 9 toy cars and needing to reach 14, framed as 9+?=149 + ? = 14. Choice C is correct because when we add 5 to 9, we get 14, so 9+5=149 + 5 = 14, which means 5 more toy cars are needed. Choice B is a common error where students add the two numbers instead of finding the difference, resulting in 23, which happens because the connection between addition and subtraction can be abstract for beginners. To help students: Use part-part-whole diagrams showing the relationship; practice on number lines with counting-on (start at 9, count forward to 14); teach fact families explicitly (9+5=149+5=14, 5+9=145+9=14, 14−9=514-9=5, 14−5=914-5=9); use 'think addition' language consistently ('to find how many more, think what plus 9 equals 14'); provide many examples connecting subtraction to unknown addend; emphasize that this strategy is especially useful when numbers are close together; use context problems where 'how many more needed' feels natural; practice writing both forms of the problem.

Question 9

13−8=?13-8=? is the same as 8+?=138+?=13. What is the missing number?

  1. 5 (correct answer)
  2. 21
  3. 8
  4. 6
Explanation: This question tests 1st grade understanding of subtraction as an unknown-addend problem (CCSS.1.OA.4). Subtraction can be solved by thinking about addition, where instead of taking away, we ask what number added to the subtrahend equals the minuend. For example, to solve 13 - 8, we can think '8 plus what equals 13?' or write it as 8 + ? = 13, and finding the missing addend (5) gives the subtraction answer. This strategy highlights the inverse relationship between addition and subtraction, making it easier for young learners to compute differences by counting on. The problem presents the subtraction 13 - 8 as equivalent to the unknown addend equation 8 + ? = 13. Choice A is correct because when we add 5 to 8, we get 13, so 8 + 5 = 13, which means 13 - 8 = 5. Choice B is a common error where students add the two numbers instead of finding the difference, resulting in 21, which happens because the connection between addition and subtraction can be abstract for beginners. To help students: Use part-part-whole diagrams showing the relationship; practice on number lines with counting-on (start at 8, count forward to 13); teach fact families explicitly (8+5=13, 5+8=13, 13-8=5, 13-5=8); use 'think addition' language consistently ('to subtract 13-8, think what plus 8 equals 13'); provide many examples connecting subtraction to unknown addend; emphasize that this strategy is especially useful when numbers are close together; use context problems where 'how many more needed' feels natural; practice writing both forms of the problem.

Question 10

Subtract 11−611-6 by finding the missing addend: 6+?=116+?=11.

  1. 6
  2. 4
  3. 5 (correct answer)
  4. 17
Explanation: This question tests 1st grade understanding of subtraction as an unknown-addend problem (CCSS.1.OA.4). Subtraction can be solved by thinking about addition, where instead of taking away, we ask what number added to the subtrahend equals the minuend. For example, to solve 11 - 6, we can think '6 plus what equals 11?' or write it as 6 + ? = 11, and finding the missing addend (5) gives the subtraction answer. This strategy highlights the inverse relationship between addition and subtraction, making it easier for young learners to compute differences by counting on. The problem presents the subtraction 11 - 6 as finding the missing addend in 6 + ? = 11. Choice C is correct because when we add 5 to 6, we get 11, so 6 + 5 = 11, which means 11 - 6 = 5. Choice D is a common error where students add the two numbers instead of finding the difference, resulting in 17, which happens because the connection between addition and subtraction can be abstract for beginners. To help students: Use part-part-whole diagrams showing the relationship; practice on number lines with counting-on (start at 6, count forward to 11); teach fact families explicitly (6+5=11, 5+6=11, 11-6=5, 11-5=6); use 'think addition' language consistently ('to subtract 11-6, think what plus 6 equals 11'); provide many examples connecting subtraction to unknown addend; emphasize that this strategy is especially useful when numbers are close together; use context problems where 'how many more needed' feels natural; practice writing both forms of the problem.

Question 11

18−11=?18-11=? is the same as 11+?=1811+?=18. What number makes it true?

  1. 7 (correct answer)
  2. 6
  3. 29
  4. 11
Explanation: This question tests 1st grade understanding of subtraction as an unknown-addend problem (CCSS.1.OA.4). Subtraction can be solved by thinking about addition, where instead of taking away, we ask what number added to the subtrahend equals the minuend. For example, to solve 18 - 11, we can think '11 plus what equals 18?' or write it as 11 + ? = 18, and finding the missing addend (7) gives the subtraction answer. This strategy highlights the inverse relationship between addition and subtraction, making it easier for young learners to compute differences by counting on. The problem presents the subtraction 18 - 11 as equivalent to the unknown addend equation 11 + ? = 18. Choice A is correct because when we add 7 to 11, we get 18, so 11 + 7 = 18, which means 18 - 11 = 7. Choice C is a common error where students add the two numbers instead of finding the difference, resulting in 29, which happens because the connection between addition and subtraction can be abstract for beginners. To help students: Use part-part-whole diagrams showing the relationship; practice on number lines with counting-on (start at 11, count forward to 18); teach fact families explicitly (11+7=18, 7+11=18, 18-11=7, 18-7=11); use 'think addition' language consistently ('to subtract 18-11, think what plus 11 equals 18'); provide many examples connecting subtraction to unknown addend; emphasize that this strategy is especially useful when numbers are close together; use context problems where 'how many more needed' feels natural; practice writing both forms of the problem.

Question 12

11−6=?11-6=? is the same as 6+?=116+?=11. What number goes in the box?

  1. 5 (correct answer)
  2. 6
  3. 4
  4. 17
Explanation: This question tests 1st grade understanding of subtraction as an unknown-addend problem (CCSS.1.OA.4). Subtraction can be solved by thinking about addition, where instead of taking away, we ask what number added to the subtrahend equals the minuend. For example, to solve 10 - 3, we can think '3 plus what equals 10?' or write it as 3 + ? = 10, and the missing addend 7 is the difference. The problem states that 11 - 6 = ? is the same as 6 + ? = 11 and asks for the number that goes in the box. Choice A is correct because when we add 5 to 6, we get 11, so 6 + 5 = 11, which means 11 - 6 = 5. Choice C is a common error where students might make an off-by-one mistake, such as counting from 6 to 11 as 4 steps instead of 5, because counting on accurately requires practice and students may include the starting number. To help students: Use part-part-whole diagrams showing the relationship; practice on number lines with counting-on (start at 6, count forward to 11); teach fact families explicitly (6+5=11, 5+6=11, 11-6=5, 11-5=6); use 'think addition' language consistently ('to subtract 11-6, think what plus 6 equals 11'); provide many examples connecting subtraction to unknown addend; emphasize that this strategy is especially useful when numbers are close together; use context problems where 'how many more needed' feels natural; practice writing both forms of the problem.

Question 13

Sofia has 6 stickers. How many more would she need in order to have 14?

  1. 7
  2. 8 (correct answer)
  3. 6
  4. 20
Explanation: This question tests 1st grade understanding of subtraction as an unknown-addend problem (CCSS.1.OA.4). Subtraction can be solved by thinking about addition, where instead of 'taking away,' we ask 'what do I add to get to the total?' For example, to solve 10−310 - 3, we can think '3 plus what equals 10?' or write it as 3+?=103 + ? = 10, and finding the missing addend (7) gives the subtraction answer. This strategy leverages the inverse relationship between addition and subtraction, making it easier for young learners to use known addition facts. The problem uses a context where Sofia has 6 stickers and asks how many more to have 14, equivalent to 14−6=?14 - 6 = ?. Choice B (8) is correct because when we add 8 to 6, we get 14, so 6+8=146 + 8 = 14, which means 14−6=814 - 6 = 8. Choice A (7) is a common error where students count incorrectly when counting on, such as an off-by-one error, which happens because counting on accurately requires practice. To help students: Use part-part-whole diagrams showing the relationship; practice on number lines with counting-on (start at smaller number, count forward to larger); teach fact families explicitly (6+8=146+8=14, 8+6=148+6=14, 14−6=814-6=8, 14−8=614-8=6); use 'think addition' language consistently ('to subtract 14−614-6, think what plus 6 equals 14'); provide many examples connecting subtraction to unknown addend; emphasize that this strategy is especially useful when numbers are close together; use context problems where 'how many more needed' feels natural; practice writing both forms of the problem.

Question 14

Anna is solving 15−915-9. She counts up from 99 to 1515. How many numbers did she count up?

  1. 2424
  2. 99
  3. 1515
  4. 66 (correct answer)
Explanation: Counting up from 9 to 15 takes 6 counts, since 9 plus 6 equals 15. 24 comes from adding 15 and 9 together instead of finding the distance between them. 9 repeats the starting number instead of counting how far she moved. 15 repeats the ending number instead of the number of counts. Only 6 correctly shows how many numbers Anna counted up from 9 to 15.

Question 15

Carlos solves 13−813-8 by covering up 88 counters out of 1313 counters. He then counts the uncovered counters to get his answer. What addition fact is Carlos finding?

  1. 8+5=138 + 5 = 13 (correct answer)
  2. 13+8=2113 + 8 = 21
  3. 5+5=105 + 5 = 10
  4. 8+8=168 + 8 = 16
Explanation: Carlos covers 8 of the 13 counters, leaving 5 uncovered, so he is finding the fact 8+5=138+5=13. 13+8=2113+8=21 adds the two numbers together instead of matching how they combine to make 13. 5+5=105+5=10 uses the wrong numbers and the wrong total. 8+8=168+8=16 repeats the covered amount instead of using the uncovered amount. Only 8+5=138+5=13 matches the counters Carlos covered and uncovered.

Question 16

20−14=?20-14=? is the same as 14+?=2014+?=20. What number goes in ??

  1. 7
  2. 14
  3. 6 (correct answer)
  4. 34
Explanation: This question tests 1st grade understanding of subtraction as an unknown-addend problem (CCSS.1.OA.4). Subtraction can be solved by thinking about addition, where instead of taking away, we ask what number added to the subtrahend equals the minuend. For example, to solve 10 - 3, we can think '3 plus what equals 10?' or write it as 3 + ? = 10, and the missing addend 7 is the difference. The problem states that 20 - 14 = ? is the same as 14 + ? = 20 and asks for the number that goes in the ?. Choice C is correct because when we add 6 to 14, we get 20, so 14 + 6 = 20, which means 20 - 14 = 6. Choice A is a common error where students might make an off-by-one mistake, such as counting from 14 to 20 as 7 instead of 6, because counting on accurately requires practice. To help students: Use part-part-whole diagrams showing the relationship; practice on number lines with counting-on (start at 14, count forward to 20); teach fact families explicitly (14+6=20, 6+14=20, 20-14=6, 20-6=14); use 'think addition' language consistently ('to subtract 20-14, think what plus 14 equals 20'); provide many examples connecting subtraction to unknown addend; emphasize that this strategy is especially useful when numbers are close together; use context problems where 'how many more needed' feels natural; practice writing both forms of the problem.

Question 17

12−912-9 is like 9+?=129+?=12. What number goes in ???

  1. 4
  2. 3 (correct answer)
  3. 21
  4. 9
Explanation: This question tests 1st grade understanding of subtraction as an unknown-addend problem (CCSS.1.OA.4). Subtraction can be solved by thinking about addition. Instead of 'taking away,' we can ask 'what do I add to get to the total?' For example, to solve 10 - 8, we can think '8 plus what equals 10?' or write it as 8 + ? = 10. Finding the missing addend (2) gives us the answer to the subtraction problem. This strategy is often easier than counting back, especially when the numbers are close together. The problem presents the subtraction 12-9 and equates it to the unknown addend equation 9 + ? = 12, asking for the missing number. Choice B is correct because when we add 3 to 9, we get 12, so 9 + 3 = 12, which means 12 - 9 = 3. Choice C is a common error where students add the two numbers instead of finding the difference, getting 21, because the connection between addition and subtraction is abstract. To help students: Use part-part-whole diagrams showing the relationship; practice on number lines with counting-on (start at smaller number, count forward to larger); teach fact families explicitly (9+3=12, 3+9=12, 12-9=3, 12-3=9); use 'think addition' language consistently ('to subtract 12-9, think what plus 9 equals 12'); provide many examples connecting subtraction to unknown addend; emphasize that this strategy is especially useful when numbers are close together; use context problems where 'how many more needed' feels natural; practice writing both forms of the problem.

Question 18

18−13=?18-13=? is the same as 13+?=1813+?=18. What is the missing number?

  1. 6
  2. 5 (correct answer)
  3. 13
  4. 31
Explanation: This question tests 1st grade understanding of subtraction as an unknown-addend problem (CCSS.1.OA.4). Subtraction can be solved by thinking about addition, where instead of taking away, we ask what number added to the subtrahend equals the minuend. For example, to solve 10 - 3, we can think '3 plus what equals 10?' or write it as 3 + ? = 10, and the missing addend 7 is the difference. The problem states that 18 - 13 = ? is the same as 13 + ? = 18 and asks for the missing number. Choice B is correct because when we add 5 to 13, we get 18, so 13 + 5 = 18, which means 18 - 13 = 5. Choice A is a common error where students might make an off-by-one mistake, such as counting from 13 to 18 as 6 instead of 5, because the connection between addition and subtraction is abstract. To help students: Use part-part-whole diagrams showing the relationship; practice on number lines with counting-on (start at 13, count forward to 18); teach fact families explicitly (13+5=18, 5+13=18, 18-13=5, 18-5=13); use 'think addition' language consistently ('to subtract 18-13, think what plus 13 equals 18'); provide many examples connecting subtraction to unknown addend; emphasize that this strategy is especially useful when numbers are close together; use context problems where 'how many more needed' feels natural; practice writing both forms of the problem.

Question 19

To subtract 15−815-8, think 8+?=158+?=15. What is ???

  1. 7 (correct answer)
  2. 23
  3. 6
  4. 8
Explanation: This question tests 1st grade understanding of subtraction as an unknown-addend problem (CCSS.1.OA.4). Subtraction can be solved by thinking about addition. Instead of 'taking away,' we can ask 'what do I add to get to the total?' For example, to solve 10 - 8, we can think '8 plus what equals 10?' or write it as 8 + ? = 10. Finding the missing addend (2) gives us the answer to the subtraction problem. This strategy is often easier than counting back, especially when the numbers are close together. The problem presents the subtraction 15-8 and suggests thinking of it as the unknown addend equation 8 + ? = 15, asking for the missing number. Choice A is correct because when we add 7 to 8, we get 15, so 8 + 7 = 15, which means 15 - 8 = 7. Choice B is a common error where students add the two numbers instead of finding the difference, getting 23, because they may be more familiar with the 'taking away' model of subtraction. To help students: Use part-part-whole diagrams showing the relationship; practice on number lines with counting-on (start at smaller number, count forward to larger); teach fact families explicitly (8+7=15, 7+8=15, 15-8=7, 15-7=8); use 'think addition' language consistently ('to subtract 15-8, think what plus 8 equals 15'); provide many examples connecting subtraction to unknown addend; emphasize that this strategy is especially useful when numbers are close together; use context problems where 'how many more needed' feels natural; practice writing both forms of the problem.

Question 20

To subtract 16−716-7, think: 7+?=167+?=16. What is the answer?

  1. 7
  2. 8
  3. 9 (correct answer)
  4. 23
Explanation: This question tests 1st grade understanding of subtraction as an unknown-addend problem (CCSS.1.OA.4). Subtraction can be solved by thinking about addition, where instead of taking away, we ask what number added to the subtrahend equals the minuend. For example, to solve 10 - 3, we can think '3 plus what equals 10?' or write it as 3 + ? = 10, and the missing addend 7 is the difference. The problem asks to subtract 16 - 7 by thinking of it as 7 + ? = 16 and finding the answer. Choice C is correct because when we add 9 to 7, we get 16, so 7 + 9 = 16, which means 16 - 7 = 9. Choice B is a common error where students might count incorrectly when counting on, such as making an off-by-one error by thinking from 7 to 16 is 8 steps instead of 9, because the unknown addend representation is less familiar and requires practice. To help students: Use part-part-whole diagrams showing the relationship; practice on number lines with counting-on (start at 7, count forward to 16); teach fact families explicitly (7+9=16, 9+7=16, 16-7=9, 16-9=7); use 'think addition' language consistently ('to subtract 16-7, think what plus 7 equals 16'); provide many examples connecting subtraction to unknown addend; emphasize that this strategy is especially useful when numbers are close together; use context problems where 'how many more needed' feels natural; practice writing both forms of the problem.