[問題]GWD 5-31

[annieway]

Q31.
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
答案是E
爬文好像沒有人問過這題..
請大家幫忙教我怎麼解
感謝~

[clbb]

1/r+1/s+1/t=1/4

1/s+1/t=1/5

得r=20

設R獨自運作需r小時完成，S、T類推獨自運作各需要s和t小時

所以R獨自運作一小時可以完成1/r

題目說R+S+T一起需要4小時完成，所以R+S+T一小時可以做1/4

式子就成了1/r+1/s+1/t=1/4

[annieway]

阿...大感謝....
我想我可能是腦袋塞住了.....Orz