How to Multiply by 99 in Your Head

How do you multiply by 99 in your head?

To multiply by 99, multiply by 100 and subtract the original number. For 43 × 99, take 4300 and subtract 43: 4300 − 43 = 4257.

It works because 99 is 100 − 1, so 43 × 99 is a hundred 43s with one 43 given back.

Subtracting in two steps keeps it easy: 4300 − 40 = 4260, then 4260 − 3 = 4257. For a two-digit number there is a pattern to read off: the first two digits are the number minus 1, and the last two digits make 100 with the number. For 43, that is 42 and 57, so 4257. For 78, it is 77 and 22, so 78 × 99 = 7722. The same idea works for 999 and any number below 1000: 125 × 999 = 125000 − 125 = 124875. Check with the digit sum, since every multiple of 99 is a multiple of 9: 4 + 2 + 5 + 7 = 18.