Numbers either side of a round number

47 × 53

The trick, step by step

1. Find the middle: 47 and 53 are both 3 away from 50.

   475053

2. Square the middle and the distance: 50² = 2500, 3² = 9.

   25009

3. Take them apart: 2500 − 9 = 2491.

   2491

given working answer taken away

Why it works

(m − d)(m + d) = m² − d², the difference of two squares: the two middle terms, −md and +md, cancel.