doing mathFree tools

Make ten: a mental math trick

When to use it: A sum crosses a ten, a hundred or a thousand.

Worked example

Making ten: 38 + 7A number line. 7 chips: 2 of them take 38 up to 40, and the other 5 carry on to 45. 38 + 7 = 45.3050404075 left+2+53845
  1. Top the bigger number up to the next ten.
  2. Take that much from the other number.
  3. Add what is left.

example 38 + 7

  1. 38 needs 2 to reach 40.
  2. 7 − 2 = 5 left.
  3. 40 + 5 = 45.

Answer: 45

Try three

  1. 77 + 7

    Show the answer

    84

  2. 16 + 7

    Show the answer

    23

  3. 17 + 8

    Show the answer

    25

Practice this in the app →

Or start with 4 warm-up problems, no sign-up →

Used in: Small facts · Gaps & place value · Two-digit add & subtract

More mental math tricks

This technique and its worked example are Doing Math’s own lesson text, checked by a blind re-solve and a review on September 27, 2026.