Missing numbers: work backwards (multi-step)

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Learn it To find a missing number in a calculation with several steps, work backwards. Undo the last thing that was done first: addition and subtraction undo each other, and multiplication and division undo each other. Always check by putting your answer back in.

Example: 3 × (□ − 4) + 5 = 20

Undo + 5: 20 − 5 = 15.
Undo 3 ×: 15 ÷ 3 = 5.
Undo − 4: 5 + 4 = 9, so □ = 9.
Check: 3 × (9 − 4) + 5 = 3 × 5 + 5 = 20 ✓

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