Mean
The arithmetic mean of a set of numbers is the most common form of average. To find the mean, first find the sum of the set of numbers and then divide the number by the number of numbers in the set.
Example 
Rosa went bowling last evening. She bowled three games. Her scores were: 
Her scores were: 
Game 1:  130 
Game 2:  124 
Game 3:  133 

What was the mean, or average, of her scores for the evening? 
130 + 124 + 133 = 387 
387 ÷ 3 = 129 

Thus, the mean of Rosa's bowling scores for the evening was 129. 
See also average.