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 ? Options.
A) 2.5 hours
B) 2 hours
C) 3.5 hours
D) 3 hours
E) None of these
Explanation:   Part filled by A in 1 hour = 1/5   Part filled by B in 1 hour = 1/10   Part filled by C in 1 hour = 1/30   Part filled by (A+B+C) in 1 hour =1/5 +1/10 +1/30   =1/3   So all pipes will fill the tank in 3 hours.   Answer is 3 hours
<result>D</result>
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 ?