Mathematics · Time and Work

There are four pipes P, Q, R, and S. Pipe P alone can fill tank A in Z hours and a drain pipe B can empty the same tank in Y hours. The rate of filling of R is 60% less than that of P, while the amount of tank A emptied by S in one hour is equal to the amount filled by P and Q together in two hours. The time taken by S alone to empty 40% of tank B is (4M + 2) hours, while the time taken by R and S together to empty tank B is 15N hours. P and Q together can fill tank A in 30 hours. Notes: a) Y - Z = 8 b) Capacity of tank B is thrice that of tank A. When all pipes are opened together for tank B, and all filling pipes work at 57 more than their usual rates, the time taken to fill tank B can be represented by which of the following expressions? I. 6N hours II. 8M hours III. 3(M + N) hours