Why are two holes made to empty an oil tin?

$A$ barometer is constructed using a liquid (density $= 760 \; kg/m^3$). What would be the height (in $m$) of the liquid column,when a mercury barometer reads $76 \; cm$? (density of mercury $= 13600 \; kg/m^3$)

Two copper vessels $A$ and $B$ have the same base area but different shapes. Vessel $A$ requires twice the volume of water as vessel $B$ to fill up to a particular common height. Which of the following statements is correct?

The pressure acting on a submarine is $3 \times 10^{5} \; Pa$ at a certain depth. If the depth is doubled,the percentage increase in the pressure acting on the submarine would be: (Assume that atmospheric pressure is $1 \times 10^{5} \; Pa$,density of water is $10^{3} \; kg \; m^{-3}$,and $g = 10 \; m \; s^{-2}$)

Explain why water storage dam walls are thicker at the bottom.

By sucking through a straw,a student can reduce the pressure in his lungs to $750\, mm$ of $Hg$ (density $= 13.6\, g/cm^3$). Using the straw,he can drink water from a glass up to a maximum depth of ....... $cm$.