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Math Help - Need help understanding this question

  1. #1
    Junior Member
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    Need help understanding this question

    The half life of Polonium-210 is 140 day and a sample of this element has a mass of 300 mg

    Find the mass that remains after t days

    y= f(t)
    initial amount is 300 mg

    f(t)= 300e^kt

    To determine the value of k we use the fact that half life is 140d and so
    f(140)= 150 thus

    300e^140k=150

    I am confused on how they get f(140) = 150?
    since f(140)=300e^140k=/= 150??
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  2. #2
    Super Member Showcase_22's Avatar
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    By the definition of half life, after 140 days the mass of the radioactive substance is half that at time 0.

    ie. f(140)=\frac{300}{2}=150
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  3. #3
    Junior Member
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    Its safe to say that:

     <br />
m(t) = \frac {300}{2^\frac {t}{140}}<br />

    Hope this helps.
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