Any number ending in 5 can be squared in your head, and the answer always ends in 25.

🪄 The trick: Take the front part, multiply it by the next number up, then stick 25 on the end.

Example 1: 35²

Front part: 3. Next number up: 4

3 × 4 = 12

Stick on 25: 35² = 1225

Example 2: 105²

Front part: 10. Next number up: 11

10 × 11 = 110

Stick on 25: 105² = 11 025

Why it works

A number ending in 5 is 10n + 5. Squaring it gives 100n² + 100n + 25 = 100n(n + 1) + 25. That is n × (n + 1) hundreds, plus 25.

Your turn

  1. 65²
    Show answer

    6 × 7 = 42, so 4225

  2. 85²
    Show answer

    8 × 9 = 72, so 7225

  3. 15²
    Show answer

    1 × 2 = 2, so 225

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