httpsdocsgooglecomfiled0B9NKCe9Ct3e3bkNYekJOSVJENE0editpref2

https://docs.google.com/file/d/0B9NKCe9Ct3e3bkNYekJOSVJENE0/edit?pref=2&pli=1

Solution

Let initial population be P0

Then P(t) = P0e^kt for a suitable k

Using the given information

P(13.81) = P0/2 = P0e^13.81k

Cancel P0 and simplify to get

1/2 = e^13.81k

Take log

-0.6931 = 13.81 k (1)

k = -0.5019

Hence P(t) = P0e^-0.5019t

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P(25) = P0 e^-0.5019(25)

P(25)/P0 = 3.558x10^-6

i.e. 0.003558% remain after 30 seconds

Decayed would be 99.986442%