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。 | 
