$2(20).$ Calculate the minimum lateral force P supporting the retaining in the following figure using Rankine's theory. Assume: Internal friction angle of the backfill soils is $25^($@$)$ and its cohesion is $0$ kPa $.$ The soil above the ground water table is dry. No frictional resistance between the wall and the backfills. The width of the wall is $20$ m $.$ Gs is $2\mathrm{.}7$ and Rw is $10$Kn$/$m$^3$