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GMAT Prep #1 - Machines type R and Type S

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veenitj Just gettin' started! Default Avatar
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GMAT Prep #1 - Machines type R and Type S Post Tue May 08, 2012 8:52 am
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  • Lap #[LAPCOUNT] ([LAPTIME])
    A compnay has two types of M/c, type R and type S. Operating at a constant rate, am/c of type R does a certain job in 36 hr and m/c type S does the sam job in 18 hr. If the copmany used the same # of each type to do this job in 2 hr, how many m/c of type R wer used?

    3
    4
    6
    9
    12

    We can easly cross out option a,b,d. I did it correct but some how I am not able to solve the problem.

    Will appreciate any replies.

    Thanks
    VJ

    OA=c

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    Post Tue May 08, 2012 9:09 am
    Each type R has a rate of 1/36, and each type S has a rate of 1/18 (or 2/36). This means that each type S machine does twice as much work as each type R machine.

    In order to complete the job in 2 hours, the combined rate of all machines must be 1/2 (or 18/36). Since each type S machine does twice as much work as each type R machine, then the total amount of work done by type S machines must be twice as much as the total amount of work done by type R machines.

    Of the combined rate of 18/36, 12/36 must be type S and 6/36 must be type R.

    Type S: 12/36 divided by the individual rate of 2/36 means that 6 type S machines were used.

    Type R: 6/36 divided by the individual rate of 1/36 means that 6 type R machines were used.

    Since the question asks about type R, the correct answer is C.

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    Post Tue May 08, 2012 10:21 am
    Quote:
    A compnay has two types of M/c, type R and type S. Operating at a constant rate, am/c of type R does a certain job in 36 hr and m/c type S does the sam job in 18 hr. If the copmany used the same # of each type to do this job in 2 hr, how many m/c of type R wer used?

    3
    4
    6
    9
    12
    # of machines used of each type = n
    Rate of R = 1/36
    Combined rate of all R machines= n/36
    Rate of S = 1/18
    Combined rate of all S machines= n/18

    Combined Rate of all machines= 1/2
    (n/36)+(n/18)=1/2
    3*n/36=1/2
    n=6
    hence answer is C[/quote]

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    Post Tue May 08, 2012 10:26 am
    To avoid fractions, plug in a value for the job. I posted a solution here:

    http://www.beatthegmat.com/gmat-prep-1-3-ps-rates-work-13-t108621.html

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    Post Tue May 08, 2012 7:40 pm
    Hi, Experts,

    I see that this assumption is very important to the problem: "If the copmany used the same # of each type to do this job"

    I wonder what will happen if such assumption is not stated. Would we need to have two equations?

    Thanks

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