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Math Help - Substitution (should be easy)

  1. #1
    Newbie
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    May 2009
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    5

    Substitution (should be easy)

    Hello,
    I have a problem where I have solved for Y and need to now solve for X. I am having trouble solving for X.

    Y = (X/2) * (PX/PY)

    X = (M/PX) - ((Py/PX) * Y))

    Subsituting Y in to the equation gives:

    X = (M/PX) - ((Py/PX) * ((X/2)*(PX/PY)))

    I am having trouble working out the steps to break this down. I wrote the answer in my notes from class as:
    X = (2M)/(3PX)

    But I'm getting stuck on working out the steps.

    Here is what I do:

    Py/PX cancels out, so you are left with:

    X = (M/PX) - (X/2)

    Then I move X to one side:

    (X/2) + X = (M/PX)

    From here I can't remember what to do. I can't tell if I wrote the answer down wrong, or if I am just rusty. I am not sure how we get to: X = (2M)/(3PX)

    Any help would be great. Thanks.
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  2. #2
    Super Member
    Joined
    May 2009
    Posts
    527
    Quote Originally Posted by lastlion View Post
    (X/2) + X = (M/PX)

    From here I can't remember what to do.
    Combine the terms on the left side. (Common denominator = 2)
    \begin{aligned}<br />
\frac{X}{2} + X &= \frac{M}{PX} \\<br />
\frac{X}{2} + \frac{2X}{2} &= \frac{M}{PX} \\<br />
\frac{3X}{2} &= \frac{M}{PX} \\<br />
\frac{2}{3}\left(\frac{3X}{2}\right) &= \frac{2}{3}\left(\frac{M}{PX}\right) \\<br />
X &= \frac{2M}{3PX}<br />
\end{aligned}


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