Rule #5: Multiples of 11 from 2 through 9 can easily be reasoned because the product is the original number twice. 11 X a = aa, or 11 X 5 = 55.

11 X 10 falls under the rule for the tens. Just put a 0 on the end of the other number.

There is also a trick for 11 X 11 and 11 X 12. You add the first number to the second number and put it between the original numbers.

Example:

11 X 11 = 1 & 1 + 1 & 1 = 121

11 X 12 = 1 & 1 + 2 & 2 = 132