When Walter drove up to the gasoline pump, he noticed that his gasoline tank was 1/8 full.  He purchased 7.5 gallons of gasoline for 10 dollars.  With this additional gasoline, his gasoline tank was then 5/8 full.  The number of gallons of gasoline his tank holds when it is full is
$\text{(A)}\ 8.75 \qquad \text{(B)}\ 10 \qquad \text{(C)}\ 11.5 \qquad \text{(D)}\ 15 \qquad \text{(E)}\ 22.5$
The tank started at $\frac{1}{8}$ full, and ended at $\frac{5}{8}$ full.  Therefore, Walter filled $\frac{5}{8} - \frac{1}{8} = \frac{4}{8} = \frac{1}{2}$ of the tank.
If Walter fills half the tank with $7.5$ gallons, then Walter can fill two halves of the tank (or a whole tank) with $7.5 \times 2 = 15$ gallons, giving an answer of $\boxed{D}$