Computer reliability. Laptop computers have been growing in popularity according to a study by Current Analysis Inc. Laptops now represent more than half the computer sales in the United States. A campus bookstore sells both types of computers and in the last semester sold 56% laptops and 44% desktops. Reliability rates for the two types of machines are quite different, however. In the first year, 5% of desktops require service, while 15% of laptops have problems requiring service.

a) Sketch a probability tree for this situation.

b) What percentage of computers sold by the bookstore last semester required service?

c) Given that a computer required service, what is the probability that it was a laptop?