AMC 8 1988 Problem 21

A fifth number, $n$, is added to the set $\{ 3,6,9,10 \}$ to make the mean of the set of five numbers equal to its median. The number of possible values of $n$ is $\text{(A)}\ 1 \text{(B)}\ 2 \text{(C)}\ 3 \text{(D)}\ 4 \text{(E)}\ \text{more than }4$