A box or question mark in a sum hides a number. In 26 + ☐ = 40, ask: what do I add to 26 to make 40? Count up from 26: 4 more makes 30, then 10 more makes 40. That is 14.
Addition and subtraction undo each other. If ☐ − 15 = 30, the missing number is 30 + 15 = 45. Use the opposite operation to find it.
Worked example
☐ + 35 = 80
- The missing number and 35 make 80.
- Take 35 away from 80: 80 − 35 = 45.
Answer:45
Key words
- missing number
- the number hidden by the box
- inverse
- the opposite operation that undoes it
- count up
- find a difference by counting forwards
Practise this
New questions every time.
Common Core-referenced: 2.OA.A.1