There are two types of associated costs, i.e. fixed cost and variable cost.
The fixed cost of renting the truck is given as $20.99, and the variable cost is the charge of per mile driven, given as 86 cents per mile (= $0.86 per mile).
Then the total cost of renting the truck is given by,
[tex]\text{Total Cost=Fixed Cost+Variable Cost}[/tex]Let 'x' be the number of miles driven.
Then the total variable cost is given by,
[tex]\begin{gathered} \text{Variable cost=No. of miles}\times\text{ Cost per mile} \\ \text{Variable cost=x}\times\text{ 0}.86 \\ \text{Variable cost=0}.86x \end{gathered}[/tex]Substitute the values,
[tex]\begin{gathered} \text{220.51=20.99+0.86x} \\ 0.86x=220.51-20.99 \\ 0.86x=199.52 \\ x=\frac{199.52}{0.86} \\ x=232 \end{gathered}[/tex]Thus, Frank drove the truck 232 miles.