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?

We know that if a number is a multiple of 9, then the sum of its digits will be divisible by 9

Sum of digits of 31z5 = 3 + 1 + z + 5 = 9 + z

Hence, 9 + z should be a multiple of 9

This is possible when 9 + z is any one of these numbers 0, 9, 18, 27, and so on …

But, since z is a single digit number, this sum can be either 9 or 18

Therefore, z should be either 0 or 9