How to Figure Denominators

Figuring math calculations in your head can be a daunting task.
Here are a few instructions for simplifying this task.

Difficulty: Moderately Easy

Instructions

1. For this article we will be dealing with whole numbers. Whole numbers do not contain a percentage, fraction of a number, or have a decimal. In other words we will be dealing with numbers above zero. i.e. 1, 2, 3, 4, 5, 6, 7...
   
   Just for knowledge's sake, natural numbers are all counting numbers that do not include a fraction both above, below, and including zero. i.e. ... -4, -3, -2, -1, 0, 1, 2, 3, 4...

2. If a number is divisible by the number 2, the number will end in a number divisible by 2. Ex: 2, 4, 6, and 8 are divisible by 2 therefore the number 743,589 is not divisible by 2 because the ending number "9" is not divisible by 2. Alternately, the number 2,436,378 is divisible by 2 because the ending number "8" is divisible by 2.
   
   Two digit numbers ending in 0 are also divisible by 2 such at 10, 20, 30, 40, 50, etc.

3. Add up multiple digit numbers and if that product is divisible by the number 3 then the original number is also divisible by the number 3. Example: 162 1+6+2= 9 since 9 is divisible by 3 then the number 162 is also divisible by 3.
   
   Another example is: 839,547 8+3+9+5+4+7=36 3+6=9 9 is divisible by 3 and therefore 839,547 is also divisible by 3.
   
   Another extension of this rule is if the number adds up to be "9" as in the 2 examples above, then the original number is also divisible by the number 9.

4. Numbers that end in either 5 or are a double digit and end in 0 are divisible by the number 5.

5. Double digit numbers that end in 0 are divisible by 10.
   
   Hope that this helps to make mathematics a happier venture for all.

[jenng]

on 7/22/2009 Great article on How to Figure Denominators 5*

[jenniferclaerr]

on 7/6/2009 Thanks for the tips on figuring denominators.