Time between two stages of decay of a sample

1. A.
   60 minutes
2. B.
   20 minutes
3. C.
   40 minutes
4. D.
   13 minutes

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1. A.
   $\frac{1}{\lambda}\ln\left( \frac{3}{2} \right)$
2. B.
   $\lambda\left( \ln\left( \frac{3}{2} \right) \right)$
3. C.
   $\frac{1}{\lambda}\ln\left( \frac{2}{3} \right)$
4. D.
   $\lambda \ln\left( \frac{2}{3} \right)$

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