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Math Help - doubling time

  1. #1
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    doubling time

    The population of the world is doubling every 35 a. In 1987 the total population reached 5 billion. find the projected world population for the year 2001.
    USE THE EXPONENTIAL GROWTH FORMULA.
    y=y_o(e)^(kt)
    35a=5e^(1987t)
    ln7a/1987=k

    y=5e^[(ln7a/1987)(2001)]
    =35.6
    this is wrong, the answer is 6.6 billion
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  2. #2
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    Quote Originally Posted by skeske1234 View Post
    The population of the world is doubling every 35 a. In 1987 the total population reached 5 billion. find the projected world population for the year 2001.
    USE THE EXPONENTIAL GROWTH FORMULA.
    y=y_o(e)^(kt)
    35a=5e^(1987t)
    ln7a/1987=k
    doubling every 35 years?

    let 1987 be t = 0

    y = 5e^{kt}

    10 = 5e^{35k}

    solve for k, then use the original equation to evaluate y when t = 14 (2001)
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  3. #3
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    Quote Originally Posted by skeeter View Post
    doubling every 35 years?

    let 1987 be t = 0

    y = 5e^{kt}

    10 = 5e^{35k}

    solve for k, then use the original equation to evaluate y when t = 14 (2001)
    no, doubling every 35 a.

    There is an a there.. and no that is not a typo because there are other questions in the textbook that read a following a number too.. I am not too sure about this eg.

    Uranium-28 has a half life of 4.5x10^9 a.
    Find the mass that remains from this sample afer 1000 a. Find the rate of decrease of the mass after 10000 a.

    So there is an a? :S
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  4. #4
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    fine ... every 35a

    let 1987 be t = 0

    y = 5e^{kt}

    10 = 5e^{(35a)k}

    solve for k in terms of a, then use the original equation to evaluate y when t = 14 (2001)
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