Given that, $1$ mole of $\mathrm{PbS}$ is oxidised by $\mathrm{X}$ moles of ${\mathrm{O}}_3$ to get $\mathrm{Y}$ moles of ${\mathrm{O}}_2.$

The balanced chemical equation can be written as,

$\mathrm{PbS}+4{\mathrm{O}}_3\to {\mathrm{PbSO}}_4+4{\mathrm{O}}_2$

Here $\mathrm{x} = 4, \mathrm{y} = 4$

Hence,

 $$\begin{gathered} \mathrm{X}+\mathrm{Y}=4+4 \\ =8 \end{gathered}$$