Question

Six machines, each working at the same constant rate, together can complete a 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

Answer 1

B. 3 more.

Explanation:

If the time for each of the machines working alone to complete the job = x hours then

6 * 1/x = 1/12

1/x = 1/12/6 = 1/72

x = 72 days.

So we have the equation

y * 1/72 = 1/8

y = 72/8 = 9.