An investment portfolio contains stocks of a large number of corporations. Over the last year the rates of return on these corporate stocks followed a normal distribution with mean 12.2% and standard deviation 7.2%.
a. For what proportion of these corporations was the rate of return higher than 20%?
b. For what proportion of these corporations was the rate of return negative?
c. For what proportion of these corporations was the rate of return between 5% and 15%?