Eloise needs to rent a car. she goes to the rental agency and finds out that she will have to pay a fee of $35 and then an additional $0.10 for every mile that she drives. how much will she have to pay if she drives 20 miles?

Well, $35 dollars no matter how many miles. if she drove 20 miles, we will say that x represents miles driven. So, the cost will be $35 + $0.10x
to solve, substitute 20 for x. 
$35 + $0.10(20)
$35 + $2
$37

Your answer is 37 dollars!