AMC 8 1996 Problem 10

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 $\text{(A)}\ 8.75 \text{(B)}\ 10 \text{(C)}\ 11.5 \text{(D)}\ 15 \text{(E)}\ 22.5$