Question

38721 divisibilty by 3 but not by 9

Answer 1

Divisibility test of 9:


Just sum up all the three digits of that number. Now if this sum is divisible by 9, then the complete number is divisible by 9 otherwise not.
For example:

1629.

Sum up all the digits of this number i.e 1+6+2+9 = 18. This gives the sum 18. Since 18 is divisible by 9 hence 1629 is also divisible by 9.