Subject: mathqa
Question: three pipes a , b and c can fill a tank from empty to full in 30 minutes , 20 minutes and 10 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 ?
Option: 
A: 6 / 14
B: 6 / 15
C: 6 / 12
D: 6 / 11
Answer: D