Multiply by 15

n × 15

The trick, step by step

1. Multiply by 10: 24 × 10 = 240.

   24× 10240

2. Halve that: half of 240 is 120.

   240÷ 2120

3. Add them: 240 + 120 = 360.

   240+120360

given working answer taken away

Why it works

15 = 10 + 5, and 5 is half of 10, so n × 15 = n × 10 + (n × 10) ÷ 2.