Multiply any number by 11!
For example, 11
1) copy down the first number
1
2) add the 2nd number to the number on its right, and put that number to the left of the first number in the answer.
21
2) copy down the end number, put that to the left
121
Taadaa.
It works for longer numbers as well, just go in steps, left to right, and add the numbers to the number on its right
14723
161953
If you come across a number >9, for example 4 + 7 = 11 in the last example, write a 1 and add a 1 to the next number you write down.
I have a whole book of these, "The Trachtenberg Speed System of Basic Mathematics" The guy is a genius.
http://www.amazon.com/exec/obidos/tg...glance&s=books
Get that book. You can work out square roots of numbers in your head in less than a minute, etc. Not even hard work, I read and understood it when I was 11.