Task: Question Answering
Input (English): Problem: an empty fuel tank is filled with brand z gasoline . when the tank is 3 / 4 empty , it is filled with brand x 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 x gasoline . at this time , what percent of the gasoline in the tank is brand z ? 
Options: a ) 41 % , b ) 42 % , c ) 43 % , d ) 43.75 % , e ) 44 %
Output (English): d