Product of a single digit number with 9

This algorithm is useful for kids that are lazy enough to memorize the multiplication table of 9.

The algorithm works like this:

Suppose you want a product of n and 9, you can do

1. n - 1


2. 10 -n


3. The answer is (n-1) as the first digit and (10 - n) as the second digit.

Example:

4 x 9 =36

First digit: 4 - 1 = 3

Second digit: 10 - 4 = 6

Note: This trick only works on multiplication of any single digit number with 9.