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Math Help - Cistern Problem

  1. #1
    Junior Member
    Joined
    Aug 2008
    From
    India
    Posts
    44

    Cistern Problem

    A cistern can be filled by taps a,b in 12 hrs and 6 hrs respectively.The full cistern can be emptied by a third tap c in 8 hrs.If all the taps be turned on at the same time, in how much time will the empty cistern be filled completly?
    (a)48 hrs
    (b)40 hrs
    (c)36 hrs
    (d)24 hrs
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  2. #2
    Newbie
    Joined
    Aug 2008
    Posts
    6

    if 24 L tank

    Lets us choose to work with a 24 L tank, as the size does not matter.
    I choose 24 as 24 = LCF(6,8,12)

    tap A adds at 2L per hour
    tap B adds at 4L per hour
    tap C empties at 3 L per hour.

    net effect is 2 + 4 - 3 = add 3 L per hour

    hence it will take 24 / 3 = 8 hours

    cheers
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