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    half life

    An unknown radioactive element decays into non-radioactive substances. In days the radioactivity of a sample decreases by percent.
    (a) What is the half-life of the element?
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    Quote Originally Posted by amiv4 View Post
    An unknown radioactive element decays into non-radioactive substances. In days the radioactivity of a sample decreases by percent.
    (a) What is the half-life of the element?
    Read this thread: http://www.mathhelpforum.com/math-he...ive-decay.html

    You have to do the same sort of thing.
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    Quote Originally Posted by amiv4 View Post
    An unknown radioactive element decays into non-radioactive substances. In days the radioactivity of a sample decreases by percent.
    (a) What is the half-life of the element?
    initial amount of substance, A_0 = x

    final amount of substance, A_N = x - 0.75x = 0.25x

    time, t = 980 days

    half life = h = ?

    A_N = A_0 \left(\frac{1}{2}\right)^{\frac{t}{h}}

    0.25x = x \left(\frac{1}{2}\right)^{\frac{980}{h}}

    0.25 = \left(\frac{1}{2}\right)^{\frac{980}{h}}

    \left(\frac{1}{2}\right)^2 = \left( \frac{1}{2} \right)^{\frac{980}{h}}

    <br />
2 = \frac{980}{h}

    h = 490 days
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