Two pipes can fill a tank in 18 minutes and 20 minutes. An outlet pipe can empty the tank in 45 minutes. If all the pipes are opened when the tank is empty, then how many minutes will it take to fill the tank?
Select
A) 1 / 15  B) 1 / 16  C) 1 / 11  D) 1 / 10  E) 1 / 12

Rationale: "part of the filled by all the three pipes in one minute = 1 / 18 + 1 / 21 - 1 / 45 = 1 / 12 so , the tank becomes full in 12 minutes . answer : e"
Answer: E