Multiply by 9

n × 9

The trick, step by step

1. Multiply by 10: 47 × 10 = 470.

   47× 10470

2. Take away one 47: 470 − 47 = 423.

   470− 47423

3. So 47 × 9 = 423.

given working answer taken away

Why it works

9 is one less than 10, so n × 9 = n × 10 − n: ten lots of the number, minus one lot.