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The decay function we need is
where A = final amount or percent of original, P = original portion, t = 1/2 life time, d = days.
From the given information, we get
solving for t we get
which is t ~ 12.7596966
so we can rewrite (i) as
The half life is ~ 12.76 days
If d = 12, we have the mass as
after 12 days, we have 52.22% left