Need help on $3\mathrm{.}93\mathrm{.}8$ Following are the standard penetration numbers determined from a sandy soil in the field. $($Note: The unit weight of soil for $0-20ft$ is $106l\frac{b}{f}{t}^3$; and is $118l\frac{b}{f}{t}^3$ from $z=20-30ft.)$

$\backslash$table$[[$Depth $($ft$),\backslash$table$[[$Unit weight of soil$],[(l\frac{b}{f}{t}^3)$