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# Six machines, each working at the same constant rate,

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Director
Joined: 11 Sep 2006
Posts: 514
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Kudos [?]: 35 [0], given: 0

Six machines, each working at the same constant rate, [#permalink]  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
Joined: 05 Feb 2006
Posts: 902
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Kudos [?]: 51 [0], given: 0

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....
SVP
Joined: 05 Jul 2006
Posts: 1519
Followers: 5

Kudos [?]: 115 [0], given: 39

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

Manager
Joined: 12 Sep 2006
Posts: 92
Followers: 1

Kudos [?]: 2 [0], given: 0

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
Joined: 30 Aug 2006
Posts: 374
Followers: 3

Kudos [?]: 20 [0], given: 0

6 machines take 12 days

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

9-6 = 3
Director
Joined: 05 Feb 2006
Posts: 902
Followers: 1

Kudos [?]: 51 [0], given: 0

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...
Senior Manager
Joined: 30 Aug 2006
Posts: 374
Followers: 3

Kudos [?]: 20 [0], given: 0

Thanks i got this wrong in the PPrep test i did so i made sure i could answer it well if i saw it again
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