Six machines, each working at the same constant rate

[GMAT 5/18]

Six machines, each working at the same constant rate, together can complete a certain job in 12 days. How many additional machines, each working at the same constant rate, will be needed to complete the job in 8 days?

a. 2
b. 3
c. 4
d. 6
e. 8

[Guest]

6 mcs = 1/12 job per day
?= 1/8 job per day

(6*1/8)/(1/12)=9mcs

Therefore 3 more than the number of mcs already operating would be required

[GMAT 5/18]

Thanks very much!