Smart Math guide · Mental Math

Square Numbers Ending in 5

For a two-digit number ending in 5, multiply the leading digit by the next integer and append 25.

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The method

How to use it

  1. Let the tens digit be n.
  2. Compute n(n+1).
  3. Append 25.
Worked example

See it in action

65²: 6×7 = 42, then append 25 → 4225.

Check your thinking

How to verify it

Estimate: 65² is a little above 60² = 3600.

Watch out

Common mistake

This shortcut as stated applies to numbers of the form 10n+5.

Keep learning

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