Question

A pipe a can fill the tank in 10 hrs, b can fill it in 20 hrs and c can empty in 40 hrs. all are opened at the same time. after how many hours shall the pipe b be closed such that the tank can be filled in 10 hrs?

Answer 1

Pipe A 1hour work= +1/10
Pipe B 1hour work=+1/20
Pipe C can empty the tank in 1 hour=-1/40
Ans-1/10+1/20-1/40=5/40=1/8 = 8hrs