A tank with vertical sides is square in plan with $3$ m long sides. The tank contains oil of relative density $0\mathrm{.}82$ to a depth of $2\mathrm{.}0$ m which is floating on water a depth of $1\mathrm{.}5$ m $.$ Calculate the force on the walls and the height of the centre of pressure from the bottom of the tank.

$(8$ marks$)$