Task: Question Answering
Input (English): Problem: an empty fuel tank is filled with brand z gasoline. when the tank is 1 / 4 empty, it is filled with brand y gasoline. when the tank is half empty again, it is filled with brand z gasoline. when the tank is half empty again, it is filled with brand y gasoline. at this time, what percent of the gasoline in the tank is brand z ?
Options: a. 31.25 %, b. 30 %, c. 40 %, d. 50 %, e. 45 %
Output (English): a