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Thread: Direct and Proportion

  1. #1
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    Direct and Proportion

    Hi,

    Im trying to do this question

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

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

    $\displaystyle = \frac{k}{7}$

    $\displaystyle = 21 X 7 $

    $\displaystyle = 147 $

    Now put 147 into the formulae with $\displaystyle x$

    $\displaystyle y = \frac{147}{13}$

    $\displaystyle = 0.something $

    Answer book says 39, how?
    Thanks
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  2. #2
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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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    Thanks. I should read the question in future
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