Junk bonds reported strong returns in 2009. The population of junk bonds earned a mean return of

Question:

Junk bonds reported strong returns in 2009. The population of junk bonds earned a mean return of 57.5% in 2009. (Data extracted from The Wall Street Journal, January 4, 2010, p. R1.) Assume that the returns for junk bonds were distributed as a normal random variable, with a mean of 57.5 and a standard deviation of 20. If you selected a random sample of 16 junk bonds from this population, what is the probability that the sample would have a mean return
a. Less than 50?
b. Between 40 and 60?
c. Greater than 40?