Pipe P can fill a tank in 6 hours, pipe Q in 12 hours and pipe R in 24 hours. If all the pipes are open, in how many hours will the tank be filled?	Select the correct option:
A) 2 hours
B) 2.4 hours
C) 3 hours
D) 3.4 hours
E) 4.5 hours
Explanation:   Part filled by (P + Q + R) in 1 hour = (1/6 + 1/12 + 1/24) = 7/24   All the three pipes together will fill the tank = 24/7 = 3.4 hours   ANSWER 3.4 hours
<result>D</result>
Pipe P can fill a tank in 6 hours, pipe Q in 12 hours and pipe R in 24 hours. If all the pipes are open, in how many hours will the tank be filled?