According to Money, in the year prior to March 2007, the average return for firms of the

Question:

According to Money, in the year prior to March 2007, the average return for firms of the S&P 500 was 13.1%. Assume that the standard deviation of returns was 1.2%. If a random sample of 36 companies in the S&P 500 is selected, what is the probability that their average return for this period will be between 12% and 15%?