c$)$ A retaining wall $6\mathrm{.}0$ m high has a smooth vertical back and retains sand having $=30,=18k\frac{N}{m^3}.$ Determine the magnitude and position of active earth pressure on the wall, if the upper surface of the backfill is horizontal and carries a uniform surcharge pressure of $20$ kPa $.$ The water table is at $3\mathrm{.}0$ m depth. Take ${}_{\text{s}\text{a}\text{t}}=20k\frac{N}{m^3}.$