Question

At time $t=0$, a radioactive sample has a mass of $10g$. Calculate the expected mass of radioactive sample after two successive mean lives.

Answer 1

Two successive mean lives $=\frac{2}{\lambda }$
Number of nuclei after two mean lives $=N_0{e}^{-\left(\lambda \right)\left(\frac{2}{\lambda }\right)}=\frac{N_0}{e^2}$
Therefore, mass $=\frac{10}{e^2}$ gram