Three printing presses, R, S, and T, working together at their respective constant rates, can do a certain printing job in 4 hours. S and T, working together at their respective constant rates, can do the same job in 5 hours. How many hours would it take R, working alone at its constant rate, to do the same job? A.
8 B.
10 C.
12 D.
15 E.
20 怎么得出的结果呀,没头绪 |