18 beavers, working together in a constant pace, can build a dam in 8 hours. How many hours will it take 36 beavers that work at the same pace, to build the same dam?  Choose the most appropriate option.
A) 2 . B) 4 . C) 5 . D) 6 E) 8 .

Rationale: "total work = 18 * 8 = 144 beaver hours 36 beaver * x = 144 beaver hours x = 144 / 36 = 4 answer : b"
Answer: B