Answer: the cost is $35.51, That must mean it would be driven 33.0 miles.
Step-by-step explanation: The given function c(r) represents the cost of a one-day truck rental when the truck is driven x miles. This means that the cost in dollars is a linear function of the number of miles driven.
To answer the first part of the question, we can plug in x = 80 into the function to find the truck rental cost when we drive 80 miles:
$$c(r) = 0.47x + 20 = 0.47(80) + 20 = \boxed{35.6}$$
To answer the second part of the question, we can solve the equation $c(r) = 0.47x + 20 = 35.51$ for x to find the number of miles driven when the cost is $35.51:
$$35.51 = 0.47x + 20 \Rightarrow 15.51 = 0.47x \Rightarrow x = \boxed{33.0}$$
Therefore, when the cost is $35.51, we must have driven 33.0 miles.