Pipe A can fill a tank in 6 hours, pipe B in 18 hours and pipe C in 36 hours. If all the pipes are open, in how many hours will the tank be filled ?
Choose the correct option.
A) 2.5 hours B) 2 hours C) 4 hours D) 3 hours E) None of these
Explanation:   Part filled by A in 1 hour = 1/6   Part filled by B in 1 hour = 1/18   Part filled by C in 1 hour = 1/36   Part filled by (A+B+C) in 1 hour =   1/6+1/18+1/36=9/36   So all pipes will fill the tank in 4 hours.   Option 4 hours
<result>C</result>
Pipe A can fill a tank in 6 hours, pipe B in 18 hours and pipe C in 36 hours. If all the pipes are open, in how many hours will the tank be filled ?