Answer
(a) $2.64\times10^{12}$
Work Step by Step
Given: $Activity=3.14\times10^{5}\,s^{-1}$
Decay constant $k=\frac{4.28\times10^{-4}}{3600\,s}=1.188889\times10^{-7}\,s^{-1}$
$Activity=kN\implies N=\frac{Activity}{k}$ where $N$ is the number of atoms.
$N=\frac{3.14\times10^{5}\,s^{-1}}{1.188889\times10^{-7}\,s^{-1}}=2.64\times10^{12}$
