A drugstore manager needs to purchase adequate supplies of various brands of toothpaste to meet the ongoing demands of its customers. In particular, the company is interested in estimating the proportion of its customers who favor the county's leading brand of toothpaste, Crest. The Data sheet of the file 'Toothpaste.xlsx' contains the toothpaste brand preferences of 200 randomly selected customers, obtained recently through customer survey.

a. Compute the proportion of customers who prefer each brand (Aquafresh, Colgate, Crest, Mentadent, and others)

Hint: You can use pivot table or 'Excel IF' function.

b. Calculate a 95% confidence interval for the proportion of customers who prefer Crest toothpaste.

c. How large should the sample be if they would like toensurethat MOE is no larger than 0.05 or 5% at 95% confidence?