An article in Tappi (1960, Vol. 43, pp. 38-44) describes an experiment that investigated the ash value of paper pulp (a measure of inorganic impurities). Two variables, temperature T in degrees Celsius and time t in hours, were studied, and some of the results are shown in the following table. The coded predictor variables shown are

and the response y is (dry ash value in %) × 103.

(a) What type of design has been used in this study? Is the design rotatable?

(b) Fit a quadratic model to the data. Is this model satisfactory?

(c) If it is important to minimize the ash value, where would you run the process?

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(T – 775) 115 (г-3) 1.5