Missing addends, step by step
A missing addend turns addition into a question about what is not there yet.
The question
15 + ? = 16
15+?=16
15+?=16
16−15=1
15+1=16
- The gap is not at the end this time — the ANSWER is already there. What is missing is one of the two numbers going in, so the sum cannot just be worked left to right.
- Adding is undone by taking away. Both numbers together make 16, and one of them is 15 — so the other is what is left when 15 is removed: 16 − 15.
- Put it back in to be sure: 15 + 1 = 16. It holds, so the missing number is 1.
How it works.
- 01
The gap is not at the end this time — the ANSWER is already there. What is missing is one of the two numbers going in, so the sum cannot just be worked left to right.
- 02
Adding is undone by taking away. Both numbers together make 16, and one of them is 15 — so the other is what is left when 15 is removed: 16 − 15.
- 03
Put it back in to be sure: 15 + 1 = 16. It holds, so the missing number is 1.