Round and Adjust

For 542 - 198, round 198 up to 200. Now 542 - 200 = 342, which you can do instantly. But you took away 2 too many, so add them back: 344.

The move is simple: turn an awkward number into a clean one, subtract, then pay back the difference. Round numbers are easy to subtract; ragged ones are not. So shape the problem before you solve it.

The same idea handles big numbers. For 8,000 - 2,347, split the second number: take away the round 2,000 first to get 6,000, then take away the leftover 347 to land on 5,653.

Breaking a subtraction into a round chunk plus a small remainder keeps every step inside your head. The rule that ties it all together: simplify before you compute.