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Math help with investments

Discussion in 'DAT Discussions' started by polishp86, Dec 21, 2008.

  1. polishp86

    polishp86 The Man, DDS
    5+ Year Member

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    Ok guys here is my question, from the topscore pro software, test one, math question 19.

    Mark has $7200 invested, part at 4% and the rest at 5%. if the annual income from both investments is the same, what is the income from his investment?

    The answer is below:






























    Let x = principle amt invested at 4%
    let 7200 - x = principle amt invested at 5%
    principle x rate = income

    .04x = .05 (7200-x)

    9x = 5(7200)

    x=4000

    4000 x .04 = income

    income = 160

    total = 160 x 2 = 320


    I get all of the above, EXCEPT for where the 9x = 5(7200) is coming from. I know it's probably some simple calculation, but I am not seeing it for the life of me, and my test is in 2 days! crap!!!!!! I also do not understand how the -x has magically disappeared after the 7200.

    anyways, I would appreciate any explanation for this, the topscore software doesn't do that great of a job explaning how it gets to certain conclusions/answers.

    thanks in advance!
     
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  3. DrYoda

    DrYoda Space Cowboy
    Physician 10+ Year Member

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    It looks like they multiplied by 100 to get the decimals as whole numbers, distributed the 5 (previously 0.05) and then moved the 5x to give 9x.

    0.04x=0.05(7200-x) multiply by 100 to get whole numbers

    4x=5(7200-x) distribute

    4x= 5*7200-5x add 5x to each side

    9x=7200*5 divide each side by 9

    x=4000
     

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