A success, s, in Bernoulli trials is often derived from a collection of outcomes. For example, an American roulette wheel consists of 38 numbers, of which 18 are red, 18 are black, and 2 are green. When the roulette wheel is spun, the ball is equally likely to land on any one of the 38 numbers. If you are interested in which number the ball lands on, each play at the roulette wheel has 38 possible outcomes. Suppose, however, that you are betting on red. Then you are interested only in whether the ball lands on a red number. From this point of view, each play at the wheel has only two possible outcomes-either the ball lands on a red number or it doesn't. Hence, successive bets on red constitute a sequence of Bernoulli trials with success probability 18 38. In four plays at a roulette wheel, what is the probability that the ball lands on red
a. Exactly twice?
b. At least once?