a) The average rate of change on that interval is 0.1
b) as we increase the speed in the interval [60, 80], the fuel economy increases at a rate of 0.1
For any function f(x), the average rate of change on an interval (a, b) is given by:
(f(b) - f(a))/(b - a)
In this case, the function is:
F(x) = −0.005x^2 + 0.8x + 12
And the interval is [60, 80]
Then we have:
F(60) =  −0.005*60^2 + 0.8*60 + 12 = 42
F(80) = −0.005*80^2 + 0.8*80 + 12 = 44
Then the average rate of change on that interval is:
R = (44 - 42)/(80 - 60) = 0.1
b) What does that rate means?
F(x) relates the fuel economy and x the speed of the vehicle.
This means that as we increase the speed in the interval [60, 80], the fuel economy increases at a rate of 0.1.
If you want to learn more about average rates of change:
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