Pipe A can fill a tank in 20 hours, pipe B in 20 hours and pipe C in 30 hours. If all the pipes are open, in how many hours will the tank be filled?	Choose the correct choice from the following answers.
A) 2  B) 2.5  C) 7  D) 7.5  E) None
Sol.   Part filled by (A + B + C) in 1 hour = (1/20 + 1/20 + 1/30) = 2/15.   âˆ´ All the three pipes together will fill the tank in 7.5 hours.   Answer 7.5
<result>D</result>
Pipe A can fill a tank in 20 hours, pipe B in 20 hours and pipe C in 30 hours. If all the pipes are open, in how many hours will the tank be filled?