can you explain the answer for Question 28 and what excel formula was used

The following chart gives the number of full years served by the first 42 presidents of the United States. (up to and including George W. Bush). Full Years Frequency 27) What is the average number of full years served by the first 42 presidents of the United States? AVERAGE 4.2 28) What is the variance of full years served by the first 42 presidents of the United States? Remember that this includes all of the presidents