Subtract by rounding

a − 29

The trick, step by step

1. Round up the number you take away: 29 is 1 less than 30.

   2930

2. Take away the round number: 83 − 30 = 53.

   53

3. You took away 1 too many, so add it back: 53 + 1 = 54.

   + 154

given working answer taken away

Why it works

a − b = a − (B − k) = a − B + k. Taking away B takes away k too many, so k goes back on.