A chain of computers stores sells three different brands of computers. Of its computer sales, 50% are brand 1 (least expensive), 30% are brand 2, and 20% are brand 3. Each manufacturer offers a 1-year warranty on parts and labor. It is known that 25% of brand 1's computers require warranty repair work, whereas the corresponding percentages for brands 2 and 3 are 20% and 10%, respectively. Let P(Ai)={brand i is purchased} and B={needs repair}. a) What is the probability that a randomly selected purchaser has bought a brand 1 computer that will need repair while under warranty? b) What is the probability that a randomly selected purchaser has a computer that will need repair while under warranty? c) If a customer returns to the store with a computer that needs warranty repair work, what is the probability that it is a brand 1 computer?