Finding the missing number

Mathematics · Addition & Subtraction · ages 7-8

Name ______________________ Date ____________

300 + ? = 550. What is the missing number?
Answer: ______________
245 + ? = 380. What is the missing number?
- 135
- 145
- 125
- 625
8 + ? = 15. What is the missing number?
- 6
- 7
- 23
To solve 4 + ? = 9, mental maths is a good choice because the numbers are small.
Circle one: True False
600 − ? = 450. What is the missing number?
Answer: ______________
Which method suits 245 + ? = 380 best?
- A written method, because the numbers are large and need borrowing
- Just guessing, because any number could work
- Counting up by ones from zero
? − 120 = 230. What is the missing number?
- 110
- 350
- 340
- 250
Eli says the missing number in 245 + ? = 380 is 145 because 245 + 145 = 380. Is Eli right?
Circle one: True False
Nadia solves ? − 38 = 24 by working out 38 − 24. What mistake did she make?
- Subtract 24 from 38 twice
- Her answer is already correct
- Add 24 and 38 instead of subtracting them
800 − ? − 150 = 400. What is the missing number?
Answer: ______________

Answer key

For grown-ups. Fold this page away before handing over the rest.

250 · The gap between 300 and 550 is 250.
135 · The gap between 245 and 380 is 135, since 245 + 135 = 380.
7 · Turn the sum around: 15 − 8 = 7, so the missing number is 7.
True · Small, friendly numbers like these are quick to work out in your head.
150 · The gap between 600 and 450 is 150.
A written method, because the numbers are large and need borrowing · Numbers with three digits and borrowing are easier to get right with a written method.
350 · The missing whole rebuilds from its parts: 230 + 120 = 350.
False · Eli is wrong. 245 + 145 = 390, overshooting 380, so the gap is 135.
Add 24 and 38 instead of subtracting them · The missing number is the total, so you add the two known numbers: 24 + 38 = 62, not 38 − 24.
250 · The two known landings cover 550 of the 800 drop, leaving 250 missing.