My favorite trick is one my father taught me after I first learned multiplication in school. The result of any two digit number mutiplied by 11 can be found by taking the sum of the two digits and inserting it between the original digits.
Example:
54 * 11 = 5(5+4)4 = 594
In case the sum is greater than 9, carry the tens digit to the first digit of the result.
Example:
56 * 11 = 5(5+6)6 = (5+1)(1)(6) = 616
I used to challenge my fellow students to see if they could find the result faster using a calculator. They thought I was some genius.
Example:
54 * 11 = 5(5+4)4 = 594
In case the sum is greater than 9, carry the tens digit to the first digit of the result.
Example:
56 * 11 = 5(5+6)6 = (5+1)(1)(6) = 616
I used to challenge my fellow students to see if they could find the result faster using a calculator. They thought I was some genius.