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by Akansha » Fri May 13, 2011 10:07 pm
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
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by Anurag@Gurome » Fri May 13, 2011 10:22 pm
Akansha wrote: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
Let x machines are needed to complete the job in 8 days.
Machines...Days
6... 12
x ... 8

6 * 12 = x * 8
x = 9 implies no. of additional machines required = 9 - 6 = 3

The correct answer is B.
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by smackmartine » Sat May 14, 2011 12:22 am
Just adding a point to Anurag's solution to clarify the relation between Machine and # of Days.

Machines...Days (inversely proportional relationship i.e more machine would complete a job in fewer # of days, so gotta multiply)
6... 12
x ... 8
6 * 12 = x * 8
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by GMATGuruNY » Sat May 14, 2011 1:32 am
Akansha wrote: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
While I would recommend Anurag's solution, another approach is to plug in a value for the job.

Let job = 72 units.
Since the job takes 12 days, work per day = 72/12 = 6 units.
Since there are 6 machines, work per machine = 6/6 = 1 unit each day.
To complete the job in 8 days, work per day = 72/8 = 9 units.
Since 9-6=3 more units must be completed each day, number of additional machines needed = 3.

The correct answer is B.
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