[求助]GWD 1-28

jojofgasw

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

Answer is E. Can anyone please tell me how to solve this question? Thank you!!

himba

设R，S，T单独完成，分别用r,s,t小时，那么速度就是1/r,1/s,1/t，由第一个画线条件得到：1/(1/r+1/s+1/t)=4 由第二个画线得到：1/（1/s+1/t)=5 即：1/s+1/t=1/5,代入前面一个方程，求出r

jojofgasw

以下是引用himba在2004-12-5 7:25:00的发言：

设R，S，T单独完成，分别用r,s,t小时，那么速度就是1/r,1/s,1/t，由第一个画线条件得到：1/(1/r+1/s+1/t)=4 由第二个画线得到：1/（1/s+1/t)=5 即：1/s+1/t=1/5,代入前面一个方程，求出r

many thanks!