4 Answers

The number three rule!

It is one of the neat things about math!  Knowing the "rules" Chelleanne posted can help you get through some timed math tests quickly.  There is a trick for multiplying by 11, too.  I have a book called "Rapid Math Tricks" and it's nice to pull a trick out once in awhile and amaze and astound family and friends.  

2 goes into even numbers, 3 goes into anything totaling a number divisible by three, 4 goes into anything that ends with 4 number divisible by four (I’m trying to remember the rule for 4).  You should study Trachtenberg math. With it you can add, multiply, square root, etc. in a matter of seconds.  19 digit numbers X 18 digit numbers in seconds!!!!! Check him here:  http://en.wikipedia.org/wiki/Trachtenberg_system