A person can swim in still water at 20 km/h. If the speed of water 12 km/h, how many hours will the man take to swim back against the current for 40km? Choose the correct choice from the following answers.
A) 3  B) 4  C) 5  D) 6  E) 7

Rationale: "m = 20 s = 12 us = 20 - 12 = 8 d = 40 t = 40 / 8 = 5 answer : c"
Answer: C