When four gallons are added to a tank that is one-third full, the tank is then one-half full.  The capacity of the tank in gallons is
$\text{(A)}\ 8 \qquad \text{(B)}\ 12 \qquad \text{(C)}\ 20 \qquad \text{(D)}\ 24 \qquad \text{(E)}\ 48$
Four gallons is $\frac{1}{2}-\frac{1}{3}= \frac{1}{6}$. Thus, capacity of the tank in gallons is $6 \times 4= \boxed{\text{(D)}\ 24}$.