Square a number ending in 5

(a5)²

The trick, step by step

1. Take the part before the 5: for 65 it is 6.

   65

2. Multiply it by the next number up: 6 × 7 = 42.

   6 × 742

3. Write 25 after it: 65² = 4225.

   4225

given working answer taken away

Why it works

(10a + 5)² = 100a² + 100a + 25 = 100 × a(a + 1) + 25, so a(a + 1) is the number of hundreds and 25 is always the end.