Determine the vertical stress, $\backslash$delta v$,$ at a depth of $15$ m when the overlying rocks include the

following $($listed in descending order from ground surface$)$: Unit A: $9$ m thick, $\backslash$gamma a $=28$

kN$/$m$3$

; Unit B: $4$ m thick, $\backslash$gamma b $=22$ kN$/$m$3$

; Unit C: $\backslash$gamma c $=25$ kN$/$m$3$

$.$