Square any number ending in 5

Multiply the front by the next number, then stick 25 on the end.

The question

What is 35²?

The trick

Take the digits before the 5, multiply them by the next whole number up, and write 25 after the result.

3×4251225

Worked example

1The front part of 35 is 3
2The next number up is 4, so 3 × 4 = 12
3Write 25 on the end: 1225

So: 1225

Remember it like this

The 5 always leaves 25 behind. The front number grabs the hand of the number after it.

In the exam

This works for three-digit numbers too: 105² → 10 × 11 = 110, so 11025. Examiners love 65² and 85² because they look hard and take ten seconds with this.

Why it works

Any number ending in 5 is (10n + 5). Squaring it gives 100n² + 100n + 25, which is 100 × n(n+1) + 25 — exactly 'front times next, then 25'.

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Try it yourself

0/5 correct

1
25²
25²
2
45²
45²
3
65²
65²
4
95²
95²
5
15²
15²