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