On pouring $15.0 \mathrm{~cm}$ of water and spirit each into the respective arms of the $\mathrm{U}$ tube, the mercury level will rise in the arm containing spirit. Let $\mathrm{h}$ be the difference in the levels of mercury in the two arms of $U$ tube and let $\rho$ be the density of mercury.
$\therefore$ The pressure exerted by $\mathrm{h} \mathrm{cm}$ of mercury column $=$ difference in pressure exerted by water and spirit.
$\therefore \quad \mathrm{h} \rho \mathrm{g}=\mathrm{h}_1 \rho_1 \mathrm{~g}-\mathrm{h}_2 \rho_2 \mathrm{~g}$
Here, $\mathrm{h}=? ; \rho=13.6 \mathrm{~g} \mathrm{~cm}^{-3} ; \mathrm{h}_1=15+10=25 \mathrm{~cm}$; $\rho_1=1 \mathrm{~g} \mathrm{~cm}^{-3} ; \mathbf{h}_2=15+12.5=27.5 \mathrm{~cm}$;
$\rho_2=0.8 \mathrm{~g} \mathrm{~cm}^{-3}$
Putting values in (i), we get
$\mathrm{h} \times 13.6 \times \mathrm{g}=25 \times 1 \times \mathrm{g}-27.5 \times 0.8 \mathrm{~g}=3 \mathrm{~g}$
or $\mathrm{h}=\frac{3}{13.6}=0.22 \mathrm{~cm}$.