请教GWD5－31

wanders

Q31:

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

Answer: E

sammen

1/(S+R+I)=4;1/(S+I)=5解个方程得，R＝1/20，所以R单独是20 小时。

wanders

是我算错了，谢谢