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Math Help - How Many Years Ago?

  1. #1
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    How Many Years Ago?

    Sean is 20 and his brother is 12. How many years ago was Sean three times as old as his brother?


    MY WORK:

    20 - x = 3(x)
    20 - x = 3x
    20 = x + 3x
    20 = 4x
    20/4 = x
    5 = x

    Sean's current age is 20.
    Sean's age some years ago is 20 - x.

    I just found x to be 5.
    So, 20 - 5 = 15.
    15 is three times more than 5.
    So, when Sean was 15 years old, his brother was 5.

    Is this correct?
    Last edited by magentarita; November 18th 2008 at 10:52 PM. Reason: Showing My Work
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  2. #2
    o_O
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    Look at your answer. If Sean's brother is 12 now, how could he have been 5 years old five years ago?

    Sean's age x years ago: {\color{red}20 - x}
    Sean's brother's age x years ago: \color{blue} 12 - x

    Sean's age x years ago was 3 times as old as Sean's brother's age x years ago.

    In equations: {\color{red}20 - x} = 3 \cdot \left({\color{blue}12 - x} \right)
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  3. #3
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    8............

    Quote Originally Posted by o_O View Post
    Look at your answer. If Sean's brother is 12 now, how could he have been 5 years old five years ago?

    Sean's age x years ago: {\color{red}20 - x}
    Sean's brother's age x years ago: \color{blue} 12 - x

    Sean's age x years ago was 3 times as old as Sean's brother's age x years ago.

    In equations: {\color{red}20 - x} = 3 \cdot \left({\color{blue}12 - x} \right)
    Is the answer 8 years ago?
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  4. #4
    o_O
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    Well plug it in.

    8 years ago, Sean was 12 and his brother was 4. Does that fit your question?
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  5. #5
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    I guess....

    Quote Originally Posted by o_O View Post
    Well plug it in.

    8 years ago, Sean was 12 and his brother was 4. Does that fit your question?
    I guess it does.
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