Rule of 99
Multiply by 100, then subtract the original.
43 × 99 = 4257
99 is 100 − 1: take a hundred 43s, then give one 43 back.
Give it back in two bites: 4300 − 40 = 4260, then 4260 − 3 = 4257.
Your turn
Three to try in your head — tap what you get.
85 × 99
55 × 99
76 × 99
All 36 shortcuts: the Magic Tricks library.