-4- 4. (Total 25 points) Sales of toothpaste seem to be heavily dependent on the level of advertisement. In order to explore this observation further, the annual advertising expenditures for several well-known brands and their annual sales were obtained as shown below. Annual advertising Annual expenditure sales Brand X y Glint 20 10 Pearl One 30 15 Shine On 50 20 Number 1 50 25 (a) (18 points) Obtain the regression equation using above data. Show the calculations to get full credit. (b) (4 points) Based on the above regression equation, an advertising expenditure of $ 75.00 should produce what amount of sales on the average? (c) (3 points) In case, for a given regression problem with a regression equation of y = 2 - 3.8x, if it is given that the correlation coefficient is -0.65, what will be the value of coefficient of determination