Question $3(20$ Marks$)$

Figure below shows U$-$Tube manometer used to measure absolute pressure of the gas in the bulb centre $"$O$".$ Mercury and water are immiscible liquids. At this unknown pressure of gas at O $,$ the interface between the liquids is at the bottom most point of the U$-$Tube. Ignore the size of tube and radius of curvature of liquids $($menisems effects & capillarity effects$).$ The height of mercury column is given as $75$ cm $.$ The pressure of gas at $O$ is : $($Neglect gas column since gas is of lower density$).$

$p=f{g}^L$