Task: Question Answering
Input (English): Problem: three pipes a , b and c can fill a tank from empty to full in 20 minutes , 20 minutes and 30 minutes respectively . when the tank is empty , all the three pipes are opened . a , b and c discharge chemical solutions p , q and r respectively . what is the proportion of solution r in the liquid in the tank after 3 minutes ? 
Options: a ) 1 / 2 , b ) 1 / 5 , c ) 1 / 3 , d ) 1 / 4 , e ) 1 / 6
Output (English): d