An investor is considering a portfolio mix of 70% of stock A and 30% of Stock B. Stock As returns are expected to be 20%, 10% and 8% in good, average and bad economies respectively. Stock Bs returns are expected to be 25%, 2% and (15%) in good, average and economies respectively. The probability of a good economy is expected to be 60%, while the average and bad economies have a 20% chance of occurrence. Given this information, calculate the standard deviation of the portfolio. please explain with proper formula's and explanation in excel