Average : The average or arithmetic mean of numbers $x_1, x_2,\ldots, x_N$ is defined as $\frac{1}{N}(x_1+x_2+\ldots+x_N).$ For example the average of 1 and 2 is $\frac{1}{2}(1+2)=1.5$ and the average of 1, 4 and 10 is $\frac{1}{3}(1+4+10)=5.$