According to a survey, one out of four investors in the United States has exchange-traded funds in their portfolios ( USA Today , 11 January 2007). Consider a random sample of 20 investors drawn from the US population.

a. Compute the probability that at least 2 investors have exchange-traded funds in their portfolios.

b. If you found that exactly 12 investors have exchange-traded funds in their portfolios, would you doubt the accuracy of the survey results?

c. Compute the expected number of investors who have exchange-traded funds in their portfolios.