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Cobalt-60 is a strong gamma emitter that has a half-life of 5.26 yr. The cobalt-60 in a radiotherapy unit must be replaced when its radioactivity falls to 75% of the original sample. If an original sample was purchased in October 2005, when will it be necessary to replace the cobalt-60?

i have no idea how to start this question please help!

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  1. after t years, the fraction left is
    2^(-t/5.26)

    so, just solve
    .75 = 2^(-t/5.26)
    t = 2.183 years

    2.183 years = 2 yrs + 2.2 months
    so, it gets tossed in Dec 2007, or Jan 2008 (if it was purchased after Oct. 25)

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