GWD1-Q28

[Karen]

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


ANS: E

THanks!

[huacy]

1. 4R+4S+4T=Const.
2. 5S+5T=Const.
-------------------------
R=1/4(S+T)

[Quinton]

1/X + 1/5 = 1/4
-->X=20

這題是OG上的基本題唷！（工時題P.65）