In the determination of the surface tension of a liquid $\mathrm{A}$ by the drop-number method, equal volumes of $\mathrm{A}$ and water gave $60$ and $20$ drops, respectively. Calculate the surface tension of $\mathrm{A}$ if $\mathrm{\rho }\left(\mathrm{A}\right)=0.86 \mathrm{g} {\mathrm{cm}}^{-3}$ and $\mathrm{\rho }\left(\mathrm{water}\right)=0.996 \mathrm{g} {\mathrm{cm}}^{-3}$. Given: ${\mathrm{\gamma }}_{\mathrm{water}}=72.75\times {10}^{-3} {\mathrm{Nm}}^{-1}$.