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 ?

A.  5/11
B.  6/11
C.  7/11
D.  8/11

Answer: B

Explanation:

Part filled by (A + B + C) in 3 minutes    = 3(1/30+1/20+1/10)
                                                              = (3×11/60)
                                                              = 11/20.
Part filled by C in 3 minutes    = 3/10.
Required ratio    = (3/10×20/11)
                          = 6/11.