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Count up to subtract: a mental math trick

When to use it: Subtracting, making change, or finding a gap.

Worked example

Counting up from 64 to 100A number line. A jump of 6 from 64 to 70, then A jump of 30 from 70 to 100. Together: 36.608090+6+3064701006 + 30 = 36
  1. Start at the smaller number.
  2. Jump to the next round number, unless the bigger number comes first.
  3. Jump on to the bigger number.
  4. Add the jumps.

example 100 − 64

  1. 64 up to 70 is 6.
  2. 70 up to 100 is 30.
  3. 6 + 30 = 36.

Answer: 36

Try three

  1. 1,000 − 192

    Show the answer

    808

  2. 100 − 79

    Show the answer

    21

  3. 100 − 83

    Show the answer

    17

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

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.