The article "The Value and the Limitations of High-Speed Turbo-Exhausters for
the Removal of Tar-Fog from Carburetted Water-Gas" (Society of Chemical
Industry Joumal \([1946]: 166-168\) ) presented data on \(y=\) tar content
(grains/100 \(\mathrm{ft}^{3}\) ) of a gas stream as a function of \(x_{1}=\)
rotor speed \((\mathrm{rev} / \mathrm{min})\) and \(x_{2}=\) gas inlet temperature
( F). A regression model using \(x_{1}, x_{2}, x_{3}=x_{2}^{2}\) and
\(x_{4}=x_{1} x_{2}\) was suggested:
$$
\begin{aligned}
\text { mean } y \text { value }=& 86.8-.123 x_{1}+5.09 x_{2}-.0709 x_{3} \\
&+.001 x_{4}
\end{aligned}
$$
a. According to this model, what is the mean \(y\) value if \(x_{1}=3200\) and
\(x_{2}=57 ?\)
b. For this particular model, does it make sense to interpret the value of any
individual \(\beta_{i}\left(\beta_{1}, \beta_{2}, \beta_{3}\right.\), or
\(\left.\beta_{4}\right)\) in the way we have previously suggested? Explain.