### Instruction: Suppose that $3$ is a factor of $a$, $a$ is a divisor of $12$, and $a$ is positive. What is the number of possible values of $a$?### Response: To find the possible values of $a$, I need to list all the divisors of $12$ and see which ones are multiples of $3$.
The divisors of $12$ are $1, 2, 3, 4, 6, 12$.
Out of these, only $3, 6, 12$ are multiples of $3$.
Therefore, there are $3$ possible values of $a$.