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Doubles and near doubles: a mental math trick

When to use it: The two numbers are the same or close together.

Worked example

Near doubles: 25 + 26Two bars, 25 and 26 long. The shared 25 of each doubles to 50; the 1 left over makes 51.2526+125 + 25 = 5050 + 1 = 51
  1. Double the smaller one.
  2. Add the difference.

example 25 + 26

  1. 25 + 25 = 50.
  2. One more: 50 + 1 = 51.

Answer: 51

Try three

  1. 17 + 18

    Show the answer

    35

  2. 42 + 43

    Show the answer

    85

  3. 43 + 44

    Show the answer

    87

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

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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.