36. According to a survey conducted by TD Ameritrade, one out of four investors have exchange-traded funds in their portfolios (USA Today, January 11, 2007). For a sample of 20 investors, answer the following questions:

a. Compute the probability that exactly four investors have exchange-traded funds in their portfolio.

b. Compute the probability that at least two of the investors have exchange-traded funds in their portfolio.

c. If you found that exactly twelve of the investors have exchange-traded funds in their portfolio, would you doubt the accuracy of the survey results?

d. Compute the expected number of investors who have exchange-traded funds in their portfolio.