An experiment was run to study the effect of two factors, time and temperature, on the inorganic impurity levels in paper pulp. The results of this experiment are shown in Table 8E.5:

a. What type of experimental design has been used in this study?

b. Fit a quadratic model to the response, using the method of least squares.

c. Construct the fitted impurity response surface. What values of x₁ and x₂ would you recommend if you wanted to minimize the impurity level?

d. Suppose that and  where temperature is in °C and time is in hours. Find the optimum operating conditions in terms of the natural variables temperature and time.

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