The following table lists the amount of discretionary spending by college students from 2008 to 2013 in billions of dollars.

a. Write the formula for the mean yearly college student discretionary spending amount over the 6-year period using sigma notation and determine that mean. Round your answer to the nearest tenth.
b. Write the formula for the mean yearly college student discretionary spending amount for the last 4 years using sigma notation and determine that mean. Round your answer to the nearest tenth.
c. Write the formula for the mean yearly college student discretionary spending for 2009 through 2011 using sigma notation and determine that mean. Round your answer to the nearest tenth.