A can write 75 pages in 25 hours. A and B together can write 135 pages in 27 hours. In what time can B write 42 pages?
Answer: C
A's writing rate = 75 pages / 25 hours = 3 pages/hour. (A+B)'s writing rate = 135 pages / 27 hours = 5 pages/hour. B's writing rate = (A+B)'s rate - A's rate = 5 - 3 = 2 pages/hour. Time for B to write 42 pages = Total pages / B's rate = 42 / 2 = 21 hours.