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 from the following:  A) 1 hr  B) 30 min  C) 15 min  D) 17 min  E) 10 min

Rationale: part filled by ( a + b + c ) in 1 hour = 1 / 10 + 1 / 20 + 1 / 40 = 7 / 40 all the 3 pipes together will fill the tank in 17 min approximately answer is d
Answer: D