Let's say you want to find out how many inventory items are not profitable (total inventory minus profitable items) or how many employees are approaching retirement age (total employees minus employees under 55). Excel gives you several ways to subtract numbers.
What do you want to do?
Subtract numbers in a cell
Use the  (minus sign) arithmetic operator. For example, if you type the following formula in a cell:
=105
The cell displays the following result:
5
Subtract numbers in a range
To do this task, use the SUM function. Adding a negative number is the same as subtracting.
Note: There is no SUBTRACT function in Excel. Use the SUM function and convert any numbers that you want to subtract to their negative values. For example, SUM(100,32,15,6) returns 77.
Example
The example may be easier to understand if you copy it to a blank worksheet.
How to copy an example

Create a blank workbook or worksheet.

Select the example in the Help topic.
Note: Do not select the row or column headers.
Selecting an example from Help 
Press CTRL+C.

In the worksheet, select cell A1, and press CTRL+V.

To switch between viewing the results and viewing the formulas that return the results, press CTRL+` (grave accent), or on the Formulas tab, in the Formula Auditing group, click the Show Formulas button.


How you use the SUM function
The SUM function adds all the numbers that you specify as arguments. Each argument can be a range, a cell reference, an array, a constant, a formula, or the result from another function. For example, SUM(A1:A5) adds all the numbers that are contained in cells A1 through A5 (a range). The range is one argument. For another example, SUM(A1, A3, A5) adds the numbers that are contained in cells A1, A3, and A5 (A1, A3, and A5 are separate arguments).