(b) de-Broglie wavelength, $\lambda = \frac{h}{\sqrt{2meV}}$
X-ray wavelength, $\lambda_{2} = \frac{hc}{eV}$
$\therefore\ \frac{\lambda_{1}}{\lambda_{2}} = \frac{eV}{c\sqrt{2meV}} = \frac{1}{c}\sqrt{\frac{1}{2}\left( \frac{e}{v} \right)V}$
$= \frac{1}{3 \times 10^{8}}\sqrt{\frac{1}{2} \times 1.8 \times 10^{11} \times 10^{4}} = \frac{1}{10}$