Professor Chernobai decided to buy a portfolio of three stocks in the following proportions: 21 percent Stock of company "X", 45 percent Stock of company "Y", and the rest of the money in Stock of company "Z". See more details on the three stocks in the table below. Calculate the standard deviation of the portfolio that Professor Chernobai bought. State of Economy Probability of State of Economy Return if State Occurs Stock X Stock Y Stock Z Normal .81 11.20% 4.60% 13.60% Boom .19 18.50% 26.50% 18.00%