$\text { Pressure of } \begin{aligned}
\mathrm{N}_2 \text { gas evolved } & =715-15 \
& =700 \mathrm{~mm} \mathrm{Hg} \
& =\frac{700}{760} \mathrm{~atm}.
\end{aligned}$
$\begin{aligned}
\therefore \text { Mole of } \mathrm{N}_2 \text { evolved } & =\frac{\mathrm{PV}}{\mathrm{RT}} \
& =\frac{700 \times 60 \times 10^{-3}}{760 \times 0.0821 \times 300} \
& =0.0022 \mathrm{~mole}
\end{aligned}$
∴wt.% of nitrogen in compound
$\begin{aligned}
& =\frac{\text { wt. of nitrogen }}{\text { wt. of compound }} \times 100 \
& =\frac{0.063}{0.4} \times 100 \
& =15.71 %
\end{aligned}$