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Math Help - Solving basic equation by cross multiplying?

  1. #1
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    Solving basic equation by cross multiplying?

    How do you go about solving this?

    200/(r+10) = (200/r) + 1

    The answer is 40.
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  2. #2
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    Re: Solving basic equation by cross multiplying?

    Quote Originally Posted by benny92000 View Post
    How do you go about solving this?
    200/(r+10) = (200/r) + 1
    The answer is 40.
    That is the same as \frac{200}{r+10}=\frac{r+200}{r}
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  3. #3
    MHF Contributor Siron's Avatar
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    Re: Solving basic equation by cross multiplying?

    Write:
    \frac{200}{r+10}=\frac{200}{r}+1 as
    \frac{200}{r+10}-\frac{200}{r}-1=0

    Now, try to make a common denominator en know that a fraction becomes zero if the numerator becomes zero, you have also to assure that r is not -10 or r is not 0, because in that case you're dividing by zero what's not defined.

    EDIT:
    I tried to solve the equation and 40 is definetly not the right answer, there're no real solutions.
    Is there a typo in it? And also, if you enter 40 int the equation you get:
    \frac{200}{50}=\frac{200}{40}+1\Leftrightarrow 4=6 which is not true!
    I think the equation has to be:
    \frac{200}{r+10}=\frac{200}{r}-1
    Last edited by Siron; August 5th 2011 at 02:30 PM.
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    Re: Solving basic equation by cross multiplying?

    Yes, a typo. It instead should read -1 instead of +1 on the right side.
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    MHF Contributor Siron's Avatar
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    Re: Solving basic equation by cross multiplying?

    Ok, but can you solve it now?
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    Re: Solving basic equation by cross multiplying?

    Yes, I can, and I did. Thank you. I think I got -50 and 40.. So I rejected negative answer (unless he were driving his car backwards ).
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  7. #7
    MHF Contributor Siron's Avatar
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    Re: Solving basic equation by cross multiplying?

    Yes, -50 and 40 are the solutions of the equation, but indeed it depends for which situation you're calculating this equation (if you have to reject a solution or not, you've to take in account a 'practical domain').
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