Near-Round Shortcuts

Multiply by 99? That is 100 minus 1, so 99 x 34 = 3,400 - 34 = 3,366. The same idea handles 25, which equals 100 divided by 4: 25 x 48 = 48 x 100 / 4 = 4,800 / 4 = 1,200. The trick is to spot when a number sits just beside a round one, then multiply by the round number and adjust. 101, 98, or 75 all yield to the same move.

Multiplying by 5 is halving, then multiplying by 10. So 5 x 86 = 86 / 2 x 10 = 43 x 10 = 430. Multiplying by 50 is the same trick scaled up: halve the other number, then multiply by 100. These patterns chain together, so once you learn a handful they combine to handle almost any multiplication you meet.