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rate problem

This topic has 1 expert reply and 2 member replies
aj5105 Legendary Member Default Avatar
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rate problem

Post Sat Jan 03, 2009 11:56 am
Elapsed Time: 00:00
  • Lap #[LAPCOUNT] ([LAPTIME])
    Three printing presses, R, S, and T, working together at their respective constant rates, can do a certain printing job in 4 hours. S and T, working together at their respective constant rates, can do the same job in 5 hours. How many hours would it take R, working alone at its constant rate, to do the same job?

    A. 8
    B. 10
    C. 12
    D. 15
    E. 20


    any quick method?



    Last edited by aj5105 on Sun Jan 04, 2009 6:59 am; edited 1 time in total

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    Post Sat Jan 03, 2009 12:14 pm
    Let R,S,T be the numbers of hours to complete the whole task individually.
    Note: after one hour, R (working alone) will have completed 1/R of the job.

    Now determine the amount completed after 1 hour for the two groups.
    1/R + 1/S + 1/T = 1/4
    1/S + 1/T = 1/5

    So, 1/R = 1/4-1/5 = 1/20
    So, R = 20

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    korkun Junior | Next Rank: 30 Posts Default Avatar
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    Test Date:
    12/26/08
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    740
    Post Sat Jan 03, 2009 2:33 pm
    lets say the work is 20x (hourxrate)


    so r,s and t together do a 5x job in 1 hour. (20/4)
    s and t together do a 4x job together in 1 hour. (20/5)

    therefore:

    r does x jobin 1 hour.
    there is 20x job.

    the answer is 20.

    never get lost in calculations. logic is always with you:)

    Post Sun Jan 04, 2009 11:16 pm
    another method,picking no,
    let total work - 60unit
    total work done in 4hrs , so 1 hr output by all three - 15 unit.
    S+T = 60/5 = 12unit work in 1 hr
    So, in 1 hr all 3 do - 15 unit
    S+t = 12 unit , that means r = 3 unit /hr

    So, total work -60 unit
    3unit /hr
    then 20 hr wil take R to complete the work alone

    It is the fastest i think it can be done in 30 sec .....i feel

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