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Manager
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Six machines, each working at the same constant rate, [#permalink]
 06 Apr 2006, 15:24
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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?
This one is easy to solve, but I'm interested in finding out in how many different ways people can solve this. Please explain your answers.
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Manager
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Well there is the easy way.
w=rt
1 = r12
1/12 = r
Each machine has a rate of 1/72.
To determine how many machines we need, let x be the number of machines.
1 = (x/72)*8
1 = x/9
x = 9
You need 9 machines to complete the job is 8 days.
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Manager
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Thanks guys 
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Intern
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Doesn't the question ask for the number of ADDITONAL machines necessary to complete the job in 8 days?
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Manager
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anbu wrote: Doesn't the question ask for the number of ADDITONAL machines necessary to complete the job in 8 days?
You are right,,I agree with you. So answer is 3 machines.
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Intern
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GMAT trap 
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VP
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I have a different approach.
If 6 machines do the work in 12 days,
to do the same work in 8 days you have to work 1.5 times as hard
meaning 6 x 1.5 = 9 machines
So the answer is 9 - 6 = 3
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Manager
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easy tips
6*12=X*8
X=9
so 3 more machine needed
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Intern
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Machines : Days : Work
6 : 12 : 1 (6 machines complete in 12 days 1 work)
? : 8 : 1 (How many machines complete in 8 days 1 work)
? = 12*6/8 = 9
Therefore, 3 more machines required.[/list]
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Manager
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Agree with 3
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Senior Manager
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Agree with 3.
One machine can complete a job in 72 days - 72*1/6=12. 72*1/x=8 x=9. So, 9-6=3. Three additional machines are needed.
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