What is mean?

Mean: The mean (or average) of observations, as we know, is the sum of the values of all the observations divided by the total number of observations. 

Suppose the observations are : 
1,2,3,4,5.

Mean       =     $\frac{1+2+3+4+5}{5}$       =  $\frac{15}{5}$    =    3


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