The idea, step by step
A box stands for a number
The equals sign means both sides have the same value. In 9 + □ = 14, the box is the missing part. Ask what must join 9 to make 14. You can count on or use 14 − 9.
Read the whole equation
In □ − 7 = 12, the box is the starting whole, not the answer to 12 − 7. Add the two known parts: 12 + 7 = 19. Put 19 back into the equation to check it.
A common trap
Always subtracting the two visible numbers, even when the starting whole is missing.
Instead: Decide whether the box is a part or the whole first. A missing whole is found by joining its parts.
See the thinking
Missing added part
Find the missing number: 8 + □ = 17.
- Count from 8 to 10: 2, then from 10 to 17: 7.
- 2 + 7 = 9; check 8 + 9 = 17.
Missing starting amount
Find the missing number: □ − 6 = 18.
- Join the remaining 18 and the removed 6.
- 18 + 6 = 24; check 24 − 6 = 18.
Check your understanding
Find □ in 25 − □ = 20.
Show answer
5
25 − 5 = 20, so the removed part is 5.
Your turn
6 questions · Work at your own pace.
Read the questions online
- Complete: 7 + □ = 12.
- Complete: □ + 9 = 16.
- Complete: 18 − □ = 10.
- Complete: □ − 5 = 22.
- Complete: 34 + □ = 40.
- Maya chooses 13 for □ + 8 = 21. Show a check.
Read the worked solutions
- Complete: 7 + □ = 12.
12 − 7 = 5.
5 - Complete: □ + 9 = 16.
16 − 9 = 7.
7 - Complete: 18 − □ = 10.
10 + 8 = 18.
8 - Complete: □ − 5 = 22.
22 + 5 = 27.
27 - Complete: 34 + □ = 40.
Count on 6 from 34 to 40.
6 - Maya chooses 13 for □ + 8 = 21. Show a check.
Substitute the chosen number into the original equation.
13 + 8 = 21, so 13 works.
Curriculum reference & lesson scope
Australian Curriculum v9 and SA Curriculum Mathematics
Facts within 20; one-/two-digit calculations; partitioning; related facts; missing numbers; checking.
Official curriculum source ↗