The Two-Digit Secret

Multiplying a two-digit number by 11 has a beautiful shortcut. For 11 times 36: split the digits (3 _ 6), add them (3 + 6 = 9), and place the sum in the middle: 396. This works for any two-digit number whose digit sum stays below 10.

When the digit sum reaches 10 or more, carry the 1 forward. For 11 times 85: split (8 _ 5), add (8 + 5 = 13), place 3 in the middle, and add 1 to the 8: 935. The carry is the only extra step, and after a few tries the whole technique becomes instant.