Explanation

(a) Mass of Uranium nucleus = mass of proton + mass of neutron.
$\ \ \ \ \ \ = 92\ \times \ 1.6725\ \times 10^{- 27}\ + \ 143\ \times \ 1.6747\ \times \ 10^{- 27})$
$= (153.87\ \ \times \ 10^{- 27}\ \ + \ 239.48\ \ \times 10^{- 27}\ )$
$= 3.93.35\ \ \times 10^{- 27}\ Kg$
since, radius of nucleus is of the order of $10^{- 15}$ m, hence, volume is
$V\ \propto \ \left( 10^{- 15} \right)^{3}\ m^{3} \propto \ 10^{- 45}\ m^{3}$
$\therefore$ Density = $\frac{mass}{volume\ } = \frac{393.35\ \times 10^{- 27}}{10^{- 45}} = \ 10^{20}\ {kgm}^{- 3}$