Neat Math Tricks to Blow Your Mind.

Want to know if 3 goes into 126?  Simple.  Add 1 + 2 + 6.  Since this equals 9, which IS divisible by 3, then so is 126.

It works just as well for the number nine. "A number is divisible by 9 if the sum of its digits is divisible by 9."  Take the the number 1098.  Is it divisible by 9?  Yup.  1 + 0 + 9 + 8 = 18, and 18 is obviously divisible by 9.

Why does that work?  'dunno.  I don't have any theorem or mathematical proof.  I suppose someone probably took the time to prove it, but I don't really care -- as long as it works.

How about this one?

What's 11 x 34?  You have five seconds to answer.

Here's the trick: Take the first number (3), then add the other two digits (3 + 4) and end with the last digit.  The answer is 374.  Don't believe me?  Do it, or break out a calculator, and try it yourself.  It also works with 3 digit numbers, but its a little trickier.

You probably already know the next one.  A quick way to determine 9 times any single digit.  Example 9 x 6.  Subtract 1 from the first digit (6-1 = 5) then figure out how many you have to add that answer to equal to 9.  In this example we need a 4.  Voila: 9 x 6 = 54.  This works just as well with any single digit.  Therefore, 9 x 3 = 27 (because 3 - 1 = 2, and 2 + 7  = 9)

Lastly, this isn't a math trick. 
Its just fun.

111,111,111 times 111,111,111 equals 12,345,678,987,654,321