How do you know if a number is divisible by 9?

Do you ever wonder if a number is divisible by other numbers?  How do you know if a number is divisible by 9?  Like the Divisibility Rule for 3 that we learned earlier this week, simply add the digits of a number, and if the sum of those digits is divisible by 9, the number is divisible by 9 as well. 
Is 792 divisible by 9?  The sum of the digits 7 + 9 + 2 = 18.  18 is divisible by 9, and you can conclude that 792 is also divisible by 9.
Let's try another one that you know is divisible by 9.  Is 99 divisible by 9?  The sum of the digits 9 + 9 = 18, and 18 is divisible by 9, therefore 99 is divisible by 9.  
Another neat fact about the Divisibility of 9, you can accurately conclude that when a number is divisible by 9, it is also divisible by 3, because 3 is a factor of 9.  Try this simple rule each and every time you need to divide a number by three or nine, and it works like magic!
Practice a few more examples on your own, and share this tip with a friend!