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6 Machines

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Source: — Problem Solving |

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by Brent@GMATPrepNow » Sat Sep 28, 2013 3:40 pm
6 machines working at a constant rate can complete a job in 12 days. How many additional machines working at the same constant rate are needed to complete the job in 8 days?

A) 2
B) 3
C) 4
D) 6
E) 8


If 6 machines take 12 days to complete the job, then we can say that the job takes 72 "machine days" to complete the job (6x12=72)

To complete the job in 8 days would require 9 machines (72 machine days divided by 8)

So, we need an additional 3 machines.

Answer = B

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by theCodeToGMAT » Sat Sep 28, 2013 7:56 pm
Work of 6 Machines = Rate x Time = 6 x 12 = 72

Work of N Machines = N x 8

Since total work is same so equating:
72 = N x 8 ==> 9
Additional Machines 9 - 6 = 3
Answer [spoiler]{B}[/spoiler]
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by vinay1983 » Sat Sep 28, 2013 10:14 pm
No.of machines is inversely proportional to the no. of days required

More machines less days

so

12/8*6= 9 machines in total are required so additional machines required are 9-6=3

B
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