A rectangular area is shown below. It is loaded with a uniform unit load of $1\mathrm{.}8$ ksf$.$ Determine the stress at the following points and depths:

$($a$)$ At a depth $4$ below the surface at Point A;

$($b$)$ At a depth $6$ below the surface at Point B; and

$($c$)$ At a depth $3$ below the surface at Point C$.$