A study was done to compare the speed $x$ (in miles per hour) with the mileage $y$ (in miles per gallon) of an automobile. The results are shown in the table. (Source: Federal Highway Administration)
$$
\begin{array}{|c|c|}
\hline \text { Speed, } \boldsymbol{x} & \text { Mileage, } \boldsymbol{y} \\
\hline 15 & 22.3 \\
20 & 25.5 \\
25 & 27.5 \\
30 & 29.0 \\
35 & 28.8 \\
40 & 30.0 \\
45 & 29.9 \\
50 & 30.2 \\
55 & 30.4 \\
60 & 28.8 \\
65 & 27.4 \\
70 & 25.3 \\
75 & 23.3 \\
\hline
\end{array}
$$
(a) Use a graphing utility to create a scatter plot of the data.
(b) Use the regression feature of a graphing utility to find a quadratic model for the data.
(c) Use a graphing utility to graph the model in the same viewing window as the scatter plot.
(d) Estimate the speed for which the miles per gallon is greatest.