The Complement Method

To subtract from a round number, use complements. The complement of 37 is 63, because 37 + 63 = 100. Notice the pattern: every digit pair sums to 9 except the last, which sums to 10. So 3+6=9 and 7+3=10.

This flips any subtraction from a power of ten into quick lookups. For 1000 - 426: each digit pairs to 9 (4+5, 2+7) and the last to 10 (6+4), giving 574. This same trick is how mechanical calculators handled subtraction for over a century.