Problem 10 (1996 AJHSME)

When Walter drove up to the gasoline pump, he noticed that his gasoline tank was $\frac{1}{8}$ full. He purchased $7.5$ gallons of gasoline for \10$. With this additional gasoline, his gasoline tank was then$\frac{5}{8}$ full. The number of gallons of gasoline his tank holds when it is full is