Q28:

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

大家如下的方法是怎么得出的呢？？？看不懂啊

1/4 - 1/5=1/20

所以＝1/(1/20)=20小时。

这种工作效率问题不是有普遍办法么？一步一步来就好了，花不了多少时间。

设R,S and T为工作效率，也就是每小时能做总工作的多少

设(S + T) = X ，反正分开没意义。用 总工作量/效率 = 时间 来列方程

1/（R+X）=4

1/（X）=5

求得R=1/20