Use the formula in terms of half life from the normal exponential functions
N(t) = N(0) (1/2) ^ (t/thalf)
N(0) is the original quantity
N(t) = quantity remaining at time t
t is the time
thalf is half life
1/16 = (1/2)^(t/3.82)
16 = 2^(t/3.82)
4 = t/3.82
t = 15.28 days
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