The smallest positive multiple of nine that is a two-digit number is $9 \times 2 = 18$,
and the largest positive multiple of nine that is a two-digit number is $9 \times 11 = 99$.
So, there are $11-2+1=10$ positive multiples of nine that are two-digit numbers.
The answer is: 10
Below is a math problem. Provide a detailed, step-by-step solution.

### Problem:
How many positive multiples of nine are two-digit numbers?

### Answer:
