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Six machines, each working at the same constant rate,

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Director
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Six machines, each working at the same constant rate, [#permalink] New post 21 Oct 2006, 09:45
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?

2
3
4
6
8
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Director
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 [#permalink] New post 21 Oct 2006, 10:42
I would choose A...

6 machines complete the job in 12 days: so each machine produce 2 something... :)

In order to complete the job in 8 days, we have to substract 4 something :)

2 additional machines would do that job....
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 [#permalink] New post 21 Oct 2006, 11:40
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?

2 3 4 6 8

each machine can do 1/12 of the job per day ie say they produce 6 choclates in 12 days

in 8 days they perform 1/12*6*8 = 4 chocklates

so we need to 3 more machines to do the extra 2 chocklates working with the others in 8 days

my answer is 3
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 [#permalink] New post 21 Oct 2006, 12:23
I also got 3

let x be the rate of the machine

6/x = 1/12

so x=72

let y be the number of machines required to finish the job in 8 days

y/72=1/8
y=9

therefore 9-6=3 machines extra
Senior Manager
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 [#permalink] New post 21 Oct 2006, 12:37
6 machines take 12 days

therefore 1 takes 72, 8 take 72/8 = 9

9-6 = 3
Director
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 [#permalink] New post 21 Oct 2006, 12:44
londonluddite wrote:
6 machines take 12 days

therefore 1 takes 72, 8 take 72/8 = 9

9-6 = 3


I knew i was making a mistake... :) This method is very good...
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 [#permalink] New post 21 Oct 2006, 12:47
Thanks :-D i got this wrong in the PPrep test i did so i made sure i could answer it well if i saw it again :lol:
  [#permalink] 21 Oct 2006, 12:47
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