Carbon11 is a radioactive isotope of carbon Its halflife is

Carbon-11 is a radioactive isotope of carbon. Its half-life is 20.3 minutes. What fraction of the initial number of carbon-11 atoms in a sample will remain after 81 minutes?
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Solution

ln(Ao/A)=kt

k=ln(2)/T

where T is the half life

so,

ln(1/k)=ln(2) * 81/20.3

or k=0.06293