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Math Help - Direct and Proportion

  1. #1
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    Sep 2008
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    Direct and Proportion

    Hi,

    Im trying to do this question

    y is directly proportional to x and y = 21 when  x = 7 . Find the value of y when  x = 13

    y = \frac{k}{x}  =   21

     = \frac{k}{7}

     =   21 X 7

     = 147

    Now put 147 into the formulae with x

     y = \frac{147}{13}

     = 0.something

    Answer book says 39, how?
    Thanks
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  2. #2
    Newbie
    Joined
    Oct 2008
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    answr

    Hi!

    The problem says it is directly proportional so:

    y=kx

    if it were inversely proportional it would be y=k/x as you wrote.

    Now if you calculate as you were doing but with the above correction you will arrive to k=3 and eventually you will obtain y=39 for x=13
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  3. #3
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    Sep 2008
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    Thanks. I should read the question in future
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