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?

Explanation

To solve the problem, we need to find the digit \( z \) in the number \( 31z5 \) such that the entire number is a multiple of 9. According to the rule of divisibility for 9, a number is a multiple of 9 if the sum of its digits is also a multiple of 9.

### Step-by-Step Solution:

1. **Identify the digits of the number**: The number is \( 31z5 \). The digits are \( 3, 1, z, \) and \( 5 \).

2. **Calculate the sum of the digits**:
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