14.14 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 Journal [1946]: 166–168) presented data on y  tar content (grains/100 ft3) of a gas stream as a function of x1  rotor speed (rev/minute) and x2 

gas inlet temperature (F). A regression model using x1, x2, x3 5 x22 and x4  x1x2 was suggested:

mean y value  86.8  .123x1  5.09x2  .0709x3

 .001x4

a. According to this model, what is the mean y value if x1  3200 and x2  57?

b. For this particular model, does it make sense to interpret the value of a b2 as the average change in tar content associated with a 1-degree increase in gas inlet temperature when rotor speed is held constant?

Explain.