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 sample of 15 outlets yielded the accompanying summary quantities.
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.

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