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

怎么得出的结果呀，没头绪

设各自做完要花R,S,T每小时，则每小时工作效率各自为1/R, 1/S, 1/T有

1、1/S+1/T+1/R=1/4;

2、1/S+1/T=1/5

连理1，2==>1/R=1/4-1/5=1/20==>R=20