Answer
\[\$340\]
Work Step by Step
First, convert all the dimensions into yards as follows:
\[\begin{align}
& \text{Length}=\frac{12feet}{1}\times \frac{1\text{ yd}}{3feet} \\
& =4\text{ yd}
\end{align}\]
\[\begin{align}
& \text{Width}=\frac{\text{9}feet}{1}\times \frac{1\text{ yd}}{3feet} \\
& =3\text{ yd}
\end{align}\]
and
\[\begin{align}
& \text{Height}=\frac{\text{6 }feet}{1}\times \frac{1\text{ yd}}{3\text{ }feet} \\
& =2\text{ yd}
\end{align}\]
Compute the volume of the rectangular solid using the equation as shown below:
\[\begin{align}
& \text{Volume of rectangular foundation}=lwh \\
& =\left( 4\text{ yd}\times 3\text{ yd}\times 2\text{ yd} \right) \\
& =24\text{ y}{{\text{d}}^{3}}
\end{align}\]
Now, compute the number of rounds of load that will be required to haul away all the dirt using the equation as shown below:
\[\begin{align}
& \text{Number of loads}=\frac{\text{Volume of the rectangular foundation}}{\text{Capacity of 1 truck}} \\
& =\frac{24\text{ y}{{\text{d}}^{3}}}{6\text{ y}{{\text{d}}^{3}}} \\
& =4
\end{align}\]
Now, compute the cost to the contractor for removing all the dirt using the equation as shown below:
\[\begin{align}
& \text{Total cost to contractor}=\text{Number of loads}\times \text{cost per load} \\
& =\text{4}\times \$\text{85}\\&=\text{}\!\!\$\!\!\text{340}\end{align}\]
