Answer:
$28.9
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The answer is D) because C) and B) are biased and would not be reasonable and A) has no guarantee of every third child being 12.
This is a strange question, and f(x) may not even exist. Why do I say that? Well..
[1] We know that f(a+b) = f(a) + f(b). Therefore, f(0+0) = f(0) + f(0). In other words, f(0) = f(0) + f(0). Subtracting, we see, f(0) - f(0) = f(0) or 0 = f(0).
[2] So, what's the problem? We found the answer, f(0) = 0, right? Maybe, but the second rule says that f(x) is always positive. However, f(0) = 0 is not positive!
Since there is a contradiction, we must either conclude that the single value f(0) does not exist, or that the entire function f(x) does not exist.
To fix this, we could instead say that "f(x) is always nonnegative" and then we would be safe.
Number......frequency.....relative frequency
10 2 2/24 = 8.3%
20 2 2/24 = 8.3%
30 4 4/24 = 16.7%
40 2 2/24 = 8.3%
50 3 3/24 = 12.5%
60 5 5/24 = 20.8%
70 3 3/24 = 12.5%
80 1 1/24 = 4.2%
90 2 2/24 = 8.3%
total : 24
(1) The train travels 4 miles per gallon.
(2) The slope of the graph is 
Explanation:
(1) The miles that train travels per gallon is given by

Dividing, we have,

Thus, the train travels 4 miles per gallon.
(2) To determine the slope, let us consider two points from the graph.
The coordinates are
and 
Thus, substituting the coordinates in the slope formula, we get,

Simplifying, we have,

Thus, the slope of the graph is 