Pipe A can fill a tank in 5 hours, pipe B in 10 hours and pipe C in 30 hours. If all the pipes are open,in how many hours will the tank be filled ?

A.  2   
B.  2.5
C.  3   
D.  3.5
Answer: C

Explanation:

part filled by (A+B+C) in 1 hour    = (1/5 + 1/6 + 1/30)
                                                     = 1/3.
All the three pipes together will fill the tank in 3 hours.