Arturo O. answered 05/23/16
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I assume the question is to find r.
Let T = the time it takes to double, given as 12 hours.
exp[r(t + T)] = 2 exp(rt)
exp(rt) * exp(rT) = 2 exp(rt)
exp(rT) = 2
rT = ln(2)
r = ln(2) / T = ln(2) / (12 hours)