Pipe A can fill a tank in 10 hr, pipe B in 20hr and pipe C in 40hr. If all the pipes are open in how many hours will tank be filled?	Choose the correct choice:
A) 1hr	B) 30min	C) 15min	D) 17min	E) 10min
Part filled by (A+B+C) in 1hour = 1/10 + 1/20 + 1/40 = 7/40   All the 3 pipes together will fill the tank in 17min Approximately   Answer is 17min
<result>D</result>
Pipe A can fill a tank in 10 hr, pipe B in 20hr and pipe C in 40hr. If all the pipes are open in how many hours will tank be filled?