[讨论]GWD5－31

xdrertty

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？？？

lololo

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

xdrertty

以下是引用lololo在2004-12-2 7:32:00的发言：

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

谢谢大牛指点！