Mental Math Shortcut

Multiplying numbers close to 100

Write each number as 100 plus or minus a small deviation, then combine the deviations instead of multiplying the full numbers.

(100 + x)(100 + y) = 10000 + 100(x + y) + xy
x and y can be negative — use the number's actual sign.

Pick a pair close to 100

deviation from 100, shown above

Now try one yourself

103 × 104