513 and 126 both are. (The question doesn't say
that there's only one of them in the list.)
Here's an easy way to tell: (I don't know how or why it works,
but it works.)
You're looking at a big number, and you need to find out if it's
divisible by 9.
Here's the two-step process:
-- If it's going to be divisible by 9, then it has to be divisible by 3.
-- Find out whether it's divisible by 3. If it is, then ...
-- Find out whether it's divisible by 9.
Step 1:
Add up all the digits in the number. If that sum is divisible by 3,
then the original big number is too.
Example:
Is 549 divisible by 3 ?
Add up its digits: (5 + 4 + 9) = 18 .
Is 18 divisible by 3 ?
Yes. That tells you that 549 is divisible by 3 .
So it could be divisible by 9.
Step 2:
Take that same 18 and add up its digits.
(1 + 8) = 9.
9 is divisible by 3.
This tells you that the original big number ... 549 ... is also divisible by 9 .
