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 12.6% and standard deviation 8.2%. a. For what proportion of these corporations was the rate of return higher than 15% ? 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 10% and 20% ? Click the icon to view the standard normal table of the cumulative distribution function. a. The proportion of corporations whose rate of return was higher than 15% is 0.3821 (Round to four decimal places as needed) b. The proportion of corporations whose rate of return was negative is (Round to four decimal places as needed.)