Q$3.(25$ p$.)$ An oil pool is separated from a pressurized water tank by means of a square$-$shaped gate $AB$ as shown in the figure below. The pressure gauge above the water tank reads $10$ kPa $.$ The specific gravity of the oil is $S.G_{\text{o}\text{i}\text{l}}=0\mathrm{.}8.$ Compute the reaction force R to keep the gate closed.

Note: $I_{xc}=\frac{1}{12}b{h}^3$