Question

If 31z5 is a multiple of 9, where z is a digit, what is the value of z? You will find that there are two answers for the last problem. Why is this so?

Answer 1

Since, 31z5 is a multiple of 9.
Therefore according to the divisibility rule of 9, the sum of all the digits should be a multiple of 9.
Therefore, 3+1+z+5=9+z

=> 9+z=9

=> z=0

Hence 0 and 9 are two possible answers.