Try the following GMAT Problem Solving question on how the average(arithmetic mean) of a data set changes when a data value is replaced by another number.
Question 54:
The average (arithmetic mean) of a list of $n$ numbers is $m$. If one of the numbers with a value of $6$ is replaced by another number of value $12$, then what is the new average for the list of $n$ numbers ?

$\quad \dfrac{m6}{n}$

$\quad \dfrac{m+6}{n}$

$\quad \dfrac{m}{n}6$

$\quad m\dfrac{6}{n}$

$\quad m+\dfrac{6}{n}$