Subtract from 100, 1000 or 10,000

1000 − n

The trick, step by step

1. Take every digit from 9, except the last: 9 − 3 = 6, 9 − 6 = 3.

   63

2. Take the last digit from 10: 10 − 7 = 3.

   3

3. So 1000 − 367 = 633.

   633

given working answer taken away

Why it works

1000 = 999 + 1. Taking a number from 999 never needs borrowing, because every digit comes from 9; the extra 1 is added to the last digit, which is why that one comes from 10.