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Thread: Why isn't my proportion working? (percentages of inputs, per volume)

  1. #1

    Why isn't my proportion working? (percentages of inputs, per volume)

    I'm 33, and I've long used proportions--kids, I promise you, you WILL use math throughout your life.

    I set up what I thought was a simple proportion, but it's not working, can anyone explain why?

    12oz beer. Xoz vodka
    ------------=-----------
    5% alcohol 40% alcohol

    12. X
    ----=------
    .05. .40

    The math doesn't get me to about 1.5oz.

    What am I doing wrong?

    Thank you!!!

  2. #2
    Elite Member
    Join Date
    Apr 2005
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    USA
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    12 / 0.05 = 240

    12 * 0.05 = 0.06

    Check your Units - ALWAYS. It's not always a division problem.
    "Unique Answers Don't Care How You Find Them." - Many may have said it, but I hear it most from me.

  3. #3
    Elite Member
    Join Date
    Sep 2012
    Posts
    2,773
    I agree with tkhunny, but I find his explanation overly condensed.

    [tex]x = \text { oz of vodka.}[/tex]

    [tex]0.05 * 12 = 0.6 \text { oz alcohol in 12 oz of 5% beer.}[/tex]

    [tex]0.4x = 0.6 \implies x = \dfrac{0.6}{0.4} = 1.5 \text { oz of 40% vodka.}[/tex]

    Percentages already are proportions, and 5% is not equal to 40%. It is easy to go wrong when you start using proportions in proportions.

    [tex]40\% = \dfrac{0.6}{1.5} \ne \dfrac{0.6}{12} = 5\%.[/tex]

    Now where tkhunny is absolutely correct is that dimensional analysis will keep you straight on many, many problems.

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