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    ktwich's Avatar
    ktwich Posts: 3, Reputation: 1
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    #1

    Oct 4, 2010, 06:15 PM
    Physics- Radioactivity
    A radioactive sample contains 10mCi of activity. 8 hours later only 2.5mCi remain. What is the half-life?
    Unknown008's Avatar
    Unknown008 Posts: 8,076, Reputation: 723
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    #2

    Oct 4, 2010, 10:50 PM

    The half life is the time a substance takes to become half its original mass, hence, half the activity.

    How many times do you have to half 10mCi to get 2.5mCi ?

    That many is the number of half lives that occurred.

    So many half lives take 8 hours.
    Find how much time 1 half life takes, and that is your value of half life.
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