Answer
$50\text{ mi}$.
Work Step by Step
Let the number of miles be $n$.
And the cost of the truck is $\$70\text{+40 cents/mile}$.
The flat fee is $\$70$.
We know that: $40\text{ cents}=\$0.4$
Then, the mileage fee is $0.4n$.
From the question, the rental cost is less than $\$90$. So,
$0.4n+70<90$ …… (1)
Solve the inequality.
$\begin{align}
& 0.4n<90-70 \\
& 0.4n<20 \\
& n<50
\end{align}$
Thus, the maximum number of miles that can be driven without going over budget is $\fbox{50 miles}$.
