To know if a number is completely divisible by 3, you can add up its digits. If its digits equal a number divisible by 3, then the number itself is divisible by 3.
9817
9 + 8 + 1 + 7
= 25
2 + 5
= 7
Since 25 isn't divisible by 3, 9817 isn't completely divisible by 3 either.
