Pipe P can fill a tank in 3 hours, pipe Q in 9 hours and pipe R in 24 hours. If all the pipes are open, in how many hours will the tank be filled? Select
A) 2 hours  B) 2.1 hours  C) 3 hours  D) 3.5 hours  E) 4.5 hours
Explanation:   Part filled by (P + Q + R) in 1 hour = (1/3 + 1/9 + 1/24) = 35/72   All the three pipes together will fill the tank = 72/35 = 2.1 hours   ANSWER 2.1 hours
<result>B</result>
Pipe P can fill a tank in 3 hours, pipe Q in 9 hours and pipe R in 24 hours. If all the pipes are open, in how many hours will the tank be filled?