The estimated probability that a Brand A, a Brand B, and a Brand C plasma TV will last at least 30,000 hr is 0.90, 0.85, and 0.80, respectively. Of the 4500 plasma TVs that Ace TV sold in a certain year, 1000 were Brand A, 1500 were Brand B, and 2000 were Brand C. If a plasma TV set sold by Ace TV that year is selected at random and is still working after 30,000 hr of use:
a. What is the probability that it was a Brand A TV?
b. What is the probability that it was not a Brand A TV?