In Excel you can calculate the average of a specific range of numbers or individual cells by using the =AVG() formula. For example you would like to know the average salary of the group of people shown in the image. You can then type =AVERAGE(C2:C6), or select the range by clicking with the left mouse button on cell C2 and while holding the button down go to cell C6 and let go. If you would only would like to know the average of a specific group of cells, you could do this by selecting the individual cells separately by holding ctrl when selecting.
Below you can find an example video where the calculation is demonstrated.