COMBIN function
This article describes the formula syntax and usage of the COMBINfunction in Microsoft Excel.
Description
Returns the number of combinations for a given number of items. Use COMBIN to determine the total possible number of groups for a given number of items.
Syntax
COMBIN(number, number_chosen)
The COMBIN function syntax has the following arguments:

Number Required. The number of items.

Number_chosen Required. The number of items in each combination.
Remarks

Numeric arguments are truncated to integers.

If either argument is nonnumeric, COMBIN returns the #VALUE! error value.

If number < 0, number_chosen < 0, or number < number_chosen, COMBIN returns the #NUM! error value.

A combination is any set or subset of items, regardless of their internal order. Combinations are distinct from permutations, for which the internal order is significant.

The number of combinations is as follows, where number = n and number_chosen = k:
where:
Example
The example may be easier to understand if you copy it to a blank worksheet.
How do I copy an example?

Select the example in this article.
Important Do not select the row or column headers.
Selecting an example from Help

Press CTRL+C.

In Excel, create a blank workbook or worksheet.

In the worksheet, select cell A1, and press CTRL+V.
Important For the example to work properly, you must paste it into cell A1 of the worksheet.

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.
After you copy the example to a blank worksheet, you can adapt it to suit your needs.

