I calculated this using the formula for volatility and got 20.32% but it is incorrect. Can someone please help me solve this?

P11-15 (similar to) Question Help Using the data from the table what is the volatility of an equally weighted portfolio of Southwest Airlines (LUV), HP Inc. (HPQ), and General Mills (GIS) stock? Hint: Make sure to round all intermediate calculations to at least four decimal places. The volatility of an equally weighted portfolio of the 3 stocks is 20.32 %. (Round to two decimal places.) X Data Table (Click on the icon located on the top-right corner of the data table below in order to copy its contents into a spreadsheet.) Historical Annual Volatilities and Correlations for Selected Stocks (based on monthly returns, 1996-2014) Southwest Ford Motor General Mills Microsoft HP Inc. Alaska Air Airlines Kellogg 20% 33% 37% 37% 31% 50% 17% Volatility (Standard Deviation) Correlation with Microsoft 0.39 0.21 0.24 0.27 0.05 0.08 1.00 0.39 HP Inc. 1.00 0.28 0.35 0.27 0.11 0.06 Alaska Air 0.21 0.28 1.00 0.39 0.15 0.15 0.20 Southwest Airlines 0.24 0.35 0.39 1.00 0.30 0.15 0.22 Ford Motor 0.27 0.27 0.15 0.30 1.00 0.18 0.06 0.05 0.11 0.15 0.15 0.18 1.00 0.54 Kellogg General Mills 0.08 0.06 0.20 0.22 0.06 0.54 1.00 Print Done Enter your answer in the answer box and then click Check