Help with finding the SD of the risk premium please

Here are stock market and Treasury bill percentage returns between 2006 and 2010 Year 2006 2007 2008 2009 2010 Stock Market Return 16.87 6.91 38.53 29.30 18.86 T-Bill Return 5.90 5.76 2.20 0.70 1.02 a. What was the risk premium on common stock in each year? (Negative values should be indicated by a minus sign. Round your answers to 2 decimal places.) Year 2006 2007 2008 2009 2010 Risk Premium 10.97 % 1.15 % -40.73 % 28.60 % 17.84 % b. What was the average risk premium? (Do not round intermediate calculations. Round your answer to 2 decimal places.) Average risk premium 357 % c. What was the standard deviation of the risk premium? (Do not round intermediate calculations. Round your answer to 2 decimal places.) Standard deviation of the risk premium