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Math Help - Half-life

  1. #1
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    Half-life

    In 660 days unknown radioactive substance decay to 33 percent of its size.

    1)What is the half life of this substance?
    t=
    2)How long will it take for a sample of 100mg to decay to 60 mg?
    T=
    not sure how to start this one
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  2. #2
    is up to his old tricks again! Jhevon's Avatar
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    Quote Originally Posted by viet View Post
    In 660 days unknown radioactive substance decay to 33 percent of its size.

    1)What is the half life of this substance?
    t=
    2)How long will it take for a sample of 100mg to decay to 60 mg?
    T=

    not sure how to start this one
    Assume that you start with 1 unit, then after 660 days, you will have 0.33 units left

    so use 0.33 = e^{-660r} to find r

    then use the formula t_h = \frac { \ln 2}{r}, where t_h is the half-life

    i think you will be able to handle the next question after getting this
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