[讨论]GWD5－31

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

可以这样计算，由于已知S与T合作，完成此工作须5小时，若ST效率相同，则各自完成任务均须10小时

设R完成须x小时

则，1/ (1/x+1/10+1/10) = 4        计算x＝20

虽然答案得出来了（有猜的嫌疑，哈哈），但是似乎对这种题目还是很模糊，希望有人告诉我正确有效的解法？？？

Your answer is right. But there is a simpler way to think about the question: assuming the total work is N, you may get two equations:

(R+S+T)*4=N;

(S+T)*5=N;

combining the two together to eliminate S and T, you can get R*20=N => 20 hours