Questions: Finding Missing Addends

Question 1

A student sees '8 + ? = 15' and says: 'I add 8 + 15 = 23, so the missing addend is 23.' What error did the student make?

• A. Nothing — 23 is correct
• B. The student added the two visible numbers instead of finding what is needed to reach the total
• C. The student should have multiplied 8 × 15 instead
• D. Missing addend problems cannot be solved when the total is 15

Answer: (B)

In a missing addend problem, 15 is the total (the whole) and 8 is one part. The job is to find the other part — not to add the part to the whole. Adding 8 + 15 treats both numbers as parts, which is the most common misconception. The correct approach is to subtract: 15 − 8 = 7.

Question 2

Which subtraction equation correctly solves the missing addend problem '6 + ? = 14'?

• A. 14 + 6 = ?
• B. 14 − 6 = ?
• C. 6 + 14 = ?
• D. 14 − ? = 14

Answer: (B)

When you know the whole (14) and one part (6), you subtract the known part from the whole to find the missing part: 14 − 6 = 8. This is the direct application of the inverse relationship between addition and subtraction.

Question 3

Finding a missing addend usually requires adding the two numbers you can see.

• T. True
• F. False

Answer: False

This is the most common error. In '5 + ? = 12,' the 12 is the total and 5 is one part — you do not add them. Instead, you subtract: 12 − 5 = 7. Adding the two visible numbers confuses 'part + part = whole' with 'part + whole = ?', which is not a meaningful equation.

Question 4

The equation '7 + ? = 13' and the subtraction equation '13 − 7 = ?' have the same answer.

• T. True
• F. False

Answer: True

Yes — both have the answer 6. Finding a missing addend is exactly the same as subtracting, because addition and subtraction are inverse operations. Knowing the whole and one part, whether written as addition or subtraction, always leads to the same calculation.

Question 5

Explain why solving '5 + ? = 12' is the same as solving '12 − 5 = ?'.

Model answer: Both ask for the same missing part. In the addition equation, you know the whole (12) and one part (5), and need the other part. Subtraction is the operation for finding a missing part when you know the whole — so subtracting the known part from the whole gives the answer. Addition and subtraction are inverse operations that undo each other.

Understanding this equivalence is more important than memorizing a rule. If you have 12 items total and 5 are on the left, the right side must hold exactly 12 − 5 = 7. The missing addend structure always models a situation where you know the total and one part, which is exactly what subtraction computes.