Pipe A can fill a tank in 5 hr, pipe B in 10hr and pipe C in 30hr. If all the pipes are open in how many hours will tank be filled?
Choose the correct choice from the following answers.
A) 1hr
B) 2hr
C) 3hr
D) 4hr
E) 5hr
Part filled by (A+B+3hr) in 1hour = 1/5 + 1/10 + 1/30 = 1/3   All the 3 pipes together will fill the tank in 3hours   Answer is 3hr
<result>C</result>
Pipe A can fill a tank in 5 hr, pipe B in 10hr and pipe C in 30hr. If all the pipes are open in how many hours will tank be filled?