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An isotope has a 12.4 hour half life. If it has an activity of 30mCi at noon today, what was its activity at 3 p.m. yesterday?

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  1. the time involved is 9+12 = 21 hours, or 1.69 half-lives

    so, we need to find P where
    30 = P*(1/2)^1.69
    P = 96.8

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