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
$\text{(A)}\ 12 \qquad \text{(B)}\ 18 \qquad \text{(C)}\ 24 \qquad \text{(D)}\ 30 \qquad \text{(E)}\ 36$
The total number of balls in the bag must be $4\times 6=24$, so there are $24-6=18$ green balls $\rightarrow \boxed{\text{B}}$