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Double and halve: a mental math trick

When to use it: One number is even and the other ends in 5.

Worked example

Double and halve: 16 × 25A 16 by 25 rectangle cut in half and laid end to end: 8 by 50, then 4 by 100. The area stays 400.162585041004 × 100 = 400
  1. Halve the even number.
  2. Double the other one.
  3. Repeat until the product is easy.

example 16 × 25

  1. 16 × 25 = 8 × 50.
  2. 8 × 50 = 4 × 100.
  3. 4 × 100 = 400.

Answer: 400

Try three

  1. 24 × 15

    Show the answer

    360

  2. 28 × 15

    Show the answer

    420

  3. 14 × 45

    Show the answer

    630

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Used in: Times tables · Bigger multiplication

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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.