Need help from the CP math whizzes

[aturnis]

Originally Posted by patteeu

They should be able to recognize that it's divisible by 3 if they know the simple "trick" that any number whose digits add up to a number divisible by 3 is, itself, divisible by 3. I'm sure you and frosty are well aware of this, but just in case, I thought I'd mention it.

2+0+0+7=9

9 is divisible by 3, therefore 2007 is divisible by 3.

Never seen this trick before. Why aren't we taught all the tricks in school? Might help.