15. Refer to the data given in Exercise 1 on tank temperature and ef ciency ratio.

a. Determine the equation of the estimated regression line.

b. Calculate a point estimate for true average ef ciency ratio when tank temperature is 182.

c. Calculate the values of the residuals from the least squares line for the four observations for which temperature is 182. Why do they not all have the same sign?

d. What proportion of the observed variation in ef ciency ratio can be attributed to the simple g gy2i

 17,454 gx2i g  39,095 g

linear regression relationship between the two variables?