11.9 A study was made by a retail merchant to determine the relation between weekly advertising expenditures and sales.

Advertising Costs ($) Sales ($)

40 385 20 400 25 395 20 365 30 475 50 440 40 490 20 420 50 560 40 525 25 480 50 510

(a) Plot a scatter diagram.

(b) Find the equation of the regression line to predict weekly sales from advertising expenditures.

(c) Estimate the weekly sales when advertising costs are $35.

(d) Plot the residuals versus advertising costs. Comment.