Give a quick stream of consciousness before answering the following question. 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
Part filled by (A + B + C) in 3 minutes = 3(1/20 + 1/20 + 1/30) = 6/15
Part filled by C in 3 minutes = 3/30
Required ratio = 3/30 * 15/6 = 1/4
The answer: (D).