OK That is a formal definition. A less formal definition of "reasonable estimate" is "at least as great as the minimum possible and no greater than the maximum possible." That is what is meant by "within the confines of the amounts given."
Does that make sense to you?
What is frequently said about estimates is whether they are "best case," "worst case," or "reasonable." A reasonable estimate is neither the lowest nor the highest possible, but somewhere in between. There is no "right" answer for an estimate. Wow, do you see how much frustration you would have avoided if you had been clear in your mind about what the problem was asking.
It was clever of you to look for information on non-uniform speed. Unfortunately, the formulas for that come from calculus. But this problem does not require even algebra. What is the minimum distance? What is the maximum distance? A reasonable estimate will fall in between. You have an arithmetic problem.
EDIT: I see there was a discussion of "arithmetic mean." Why is that a common way to find a reasonable estimate?