Solubility product of a sparingly soluble salt ${\mathrm{P}}_2\mathrm{Q}$ is $4\times {10}^{-15}{\mathrm{M}}^3.$  $\left[{\mathrm{P}}^{+}\right]$   in its saturated solution is

$$\begin{gathered} {\mathrm{P}}_2\mathrm{Q}\underset{ }{\rightleftharpoons }\underset{2\mathrm{s}}{2{\mathrm{P}}^{+}}+\underset{\mathrm{s}}{{\mathrm{Q}}^{-2}} \\ {\mathrm{k}}_{\mathrm{SP}}={\left(2\mathrm{s}\right)}^2{\left(\mathrm{s}\right)}^1 \\ {\text{= 4s}}^{\text{3}} \\ 4{\mathrm{s}}^3=4\times {10}^{-15} \\ \boxed{ \mathrm{s}={10}^{-5}\mathrm{M}} \\ \therefore \left[{\mathrm{Q}}^{-2}\right]={10}^{-5}\mathrm{M} \\ \left[{\mathrm{P}}^{+}\right]=2\times {10}^{-5}\mathrm{M} \end{gathered}$$