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? Answers.
A) 1 / 2 B) 1 / 5 C) 1 / 3 D) 1 / 4 E) 1 / 6

Rationale: 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 answer : d
Answer: D