$\begin{array}{1 1} P^{\circ}-P_s/P^{\circ}=\frac{n}{N} \\ P^{\circ}-P_s/P^{\circ}=\frac{N}{n} \\ P^{\circ}- P_s/P_s=\frac{n}{N} \\ P^{\circ} - P_s/P^{\circ}=n \times N \end{array} $
