13.16 A simple linear regression model was used to describe the relationship between sales revenue y (in thousands of dollars) and advertising expenditure x (also in thousands of dollars) for fast-food outlets during a 3-month period. A random sample of 15 outlets yielded the accompanying summary quantities.

o x 5 14.10 o y 5 1438.50 o x2 5 13.92 o y2 5 140,354 o xy 5 1387.20 o sy 2 yd2 5 2401.85 o sy 2y⁄

d2 5 561.46

a. What proportion of observed variation in sales revenue can be attributed to the linear relationship between revenue and advertising expenditure?

b. Calculate se and sb.

c. Obtain a 90% confidence interval for

b, the average change in revenue associated with a $1000

(that is, 1-unit) increase in advertising expenditure.