AMC 8 1990 Problem 14

A bag contains only blue balls and green balls. There are $6$ blue balls. If the probability of drawing a blue ball at random from this bag is $\frac{1}{4}$, then the number of green balls in the bag is $\text{(A)}\ 12 \text{(B)}\ 18 \text{(C)}\ 24 \text{(D)}\ 30 \text{(E)}\ 36$