This is going to be a fascinating math fact. I’ve written about it before. It was on my old, long gone blog, which I deleted several years ago. So, I thought I would repeat it here. As a designer, this fun fact has come in handy many times.
Did you know…?
You can determine whether a number is divisible by the number three just by looking at it, no matter how big the number.
Let’s take the number 498. I can tell immediately whether this number is divisible by three. Simply add all the digits. 4+9+8 = 21. If the resulting answer is divisible by three, then the original number is divisible by three.
The Rule of 3s works with any number. Isn’t that fun?
As far as I know, this doesn’t work for any other number. It would be cool if it did. Try it out! You never know when this information might come in handy.
ETA: Please see comments below. This also works for the number 9. Very cool! 
Since 9 is 3-squared, I wonder whether it also works for 3-cubed (27), but perhaps that isn’t nearly as handy as 3 and 9.
December 5, 2012 at 4:22 pm
Kim, I was excited to read your mathy post. I’m a math professor and love it when people get excited about math. You mentioned that you weren’t sure if there were any other numbers that work like 3. Indeed there are! 9 works exactly the same. So a number is divisible by 9 If the sum of the digits is divisible by 9. For example, 468 is divisible by 9 since 4+6+8=18. There are other divisibility tests for small numbers (mostly single digit numbers), but I always thought the tests for 3 and 9 were the coolest.
December 11, 2012 at 12:45 am
Thank you for that! Very interesting.
December 6, 2012 at 8:51 am
I like the rule for six. If a number is divisible by 2 (ends in 0 or 2) and 3 then it’s divisible by 6.
December 11, 2012 at 12:45 am
Thank you for the information!