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How many machines of type R???

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by gmatpup » Tue Dec 27, 2011 10:21 am
A company has two machines, type R and type S. Operating at a constant rate, a machine of type R does the same job in 18 hours. If the company used the same number of each to do the job in 2 hours, how many machines of type R were used?

A. 3
B. 4
C. 6
D. 9
E. 12

Answer is C

Please show me how to do this type of problem. Thanks so much!
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by sam2304 » Tue Dec 27, 2011 10:38 am
Is the question right ? Or am i missing something here.
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by user123321 » Tue Dec 27, 2011 11:05 am
gmatpup wrote:A company has two machines, type R and type S. Operating at a constant rate, a machine of type R does the same job in 18 hours. If the company used the same number of each to do the job in 2 hours, how many machines of type R were used?

A. 3
B. 4
C. 6
D. 9
E. 12

Answer is C

Please show me how to do this type of problem. Thanks so much!
There is something which is missing in the question.
please verify the question.

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by pemdas » Tue Dec 27, 2011 11:11 am
R's speed 1/18, S speed is unknown
with one machine R and one machine S, combined speed is (S+18)/18S
with x no of machines R and S, combined speed is 1/2
Hence, x(S+18)/18S=1/2 OR 2x=18S/(S+18), x=9S/(S+18)

x can be only integer
x=1 -> 9S=S+18, S=18/8
x=2 -> 9S=2S+36, S=36/7


something missing
gmatpup wrote:A company has two machines, type R and type S. Operating at a constant rate, a machine of type R does the same job in 18 hours. If the company used the same number of each to do the job in 2 hours, how many machines of type R were used?

A. 3
B. 4
C. 6
D. 9
E. 12

Answer is C

Please show me how to do this type of problem. Thanks so much!
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by gmatpup » Tue Dec 27, 2011 11:57 am
Oh my goodness I am so sorry, I totally missed a sentence :|!!


Here is the question:

A company has two machines, type R and type S. Operating at the same constant rate, a machine of type R does a job in 36 hours and a machine of type S does the same job in 18 hours. If the company used the same number of each to do the job in 2 hours, how many machines of type R were used?

A. 3
B. 4
C. 6
D. 9
E. 12

Answer is C

I apologize!
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by pemdas » Tue Dec 27, 2011 12:41 pm
plugging new info: R's speed is 1/36, S speed is 1/18
with one machine R and one machine S, combined speed is 1/36+1/18=1/12
with x no of machines R and S, combined speed is 1/2
Hence, x(1/12)=1/2, x=6

no of machines R and S is 6 of each
pemdas wrote:R's speed 1/18, S speed is unknown
with one machine R and one machine S, combined speed is (S+18)/18S
with x no of machines R and S, combined speed is 1/2
Hence, x(S+18)/18S=1/2 OR 2x=18S/(S+18), x=9S/(S+18)

x can be only integer
x=1 -> 9S=S+18, S=18/8
x=2 -> 9S=2S+36, S=36/7

something missing
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by Anurag@Gurome » Tue Dec 27, 2011 8:43 pm
gmatpup wrote:Oh my goodness I am so sorry, I totally missed a sentence :|!!


Here is the question:

A company has two machines, type R and type S. Operating at the same constant rate, a machine of type R does a job in 36 hours and a machine of type S does the same job in 18 hours. If the company used the same number of each to do the job in 2 hours, how many machines of type R were used?

A. 3
B. 4
C. 6
D. 9
E. 12

Answer is C

I apologize!

In 1 hour, machine R does 1/36th of the job

In 1 hour, machine S does 1/18th of the job

In 1 hour, machine R and S together does 1/36 + 1/18 = 3/36 = 1/12th of the job or R + S together take 12 hours to do 1 job.

Now if the job is to be done in 2 hours, then we'll need 12/2 = 6 machines each.

The correct answer is C.
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