One pipe can fill a tank four times as fast as another pipe. If together the two pipes can fill the tank in 40 minutes, then the slower pipe alone will be able to fill the tank in?	Choose the correct choice from the following choices.
A) 100 min  B) 150 min  C) 200 min  D) 250 min  E) 300 min

Rationale: "let the slower pipe alone fill the tank in x minutes then , faster pipe will fill it in x / 4 minutes 1 / x + 4 / x = 1 / 40 5 / x = 1 / 40 x = 200 min answer is c"
Answer: C