b
\(\mathrm{t}_{1 / 2}=20 \mathrm{min}\)

\(\lambda=\frac{\ell \mathrm{n} 2}{20}\)

\(\frac{0.8 \mathrm{N}_{0}=\mathrm{N}_{0} \mathrm{e}^{-\lambda t_{1}}}{0.2 \mathrm{N}_{0}=\mathrm{N}_{0} \mathrm{e}^{-\lambda\left(t_{2}\right)}}\)

\(4=e^{+\lambda\left(t_{2}-t_{1}\right)}\)

\(\ln 4=\lambda\left(t_{2}-t_{1}\right)\)

\(\left(t_{2}-t_{1}\right)=40 \mathrm{\,min}\)