Test Prep I -- PS 13

[lisayang83]

Machine R can do a certain job in 36 hours, and machine S can do th same job in 18 hours. If the same number of machine r and s worked together and finished a certain job in 2 hours, how many machine R were used?

Ans. 6

[dibert8]

speed of R machine = 1/36
speed of S machine = 1/18
assume x machines each of R and S to work for 2 hr,
2 * (x/36 + x/18) = 1 => x = 6

[lisayang83]

謝謝
所以 做這種題目時 不可以 自作聰明的 把 36 小時 與 18 小時 化減為 2 與 1 ><"