The graph you described represents the cost of Jasper's rental truck as a function of the number of hours he rents it.
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The flat fee of $25 represents the starting point (the y-intercept) on the graph when the number of hours (x-value) is 0. This means that no matter how many hours Jasper rents the truck, he will always incur this initial charge.
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The hourly charge of $15 represents the slope of the line. This means that for every hour Jasper rents the truck, the total cost increases by $15.
The rate of change in this context is the hourly charge, which is $15.
So the correct answer to the question about the rate of change is:
15