Working alone at its constant rate, machine K took 3 hours to produce 1/4 of the units produced last Friday.  Then machine M started working and the two machines, working simultaneously at their respective constant rates, took 6 hours to produce the rest of the units produced last Friday.  How many hours would it have taken machine M, working alone at its constant rate, to produce all of the units produced last Friday?

 A.      8

 A.      8

B.      12

C.      16

D.     24

E.      30

Answer: d

请教这题的思路。谢谢。算出来是12，不知如何得出24。

K的生产效率是1/4/3＝1/12 unit/hour

设M的生产效率为M,有

6(1/12+M)=3/4

M=1/24

所以it have taken machine M, working alone at its constant rate, to produce all of the units produced last Friday：

1/（1/24）＝24