39 is divisible by 3. It equals 13. You can tell if a number is
divisible by 3 by adding up the digits in the number and see if it
is divisible by three. Here's and example:
197
1+9+7= 17 and 17 isn't divisible by three.
576
5+7+6= 18 and 3 goes into 18 6 times so it is divisible by 3