hii

First of all note that the temperature of gas remains constant, so we have

                                P₁V₁ = P₂V₂

 

Now initially the the pressure in atmospheric pressure only, so let it be P

This is also the pressure of the gas.

                    So, P₁V₁ =  PA.(90)   where A is the area of the cylinder

 

When we start pouring the mercury the disc goes down by 32 cm leaving a space of 32 + 10 = 42 cm in the cylinder because the piston was at 90 cm initially.

 

So now the total pressure from outside is P + 42 which is equal to pressure of the gas.   So,  P₂V₂ = ( P + 42 ).A.58

 

                   So using P₁V₁ = P₂V₂

                          we get PA.(90) = ( P + 42 ).A.58

                    and thus solving for P we get P = 76.125 cm of mercury.