COMBINA function
This article describes the formula syntax and usage of the COMBINA function in Microsoft Excel.
Description
Returns the number of combinations (with repetitions) for a given number of items.
Syntax
COMBINA(number, number_chosen)
The COMBINA function syntax has the following arguments.

Number Required. Must be greater than or equal to 0, and greater than or equal to Number_chosen. Noninteger values are truncated.

Number_chosen Required. Must be greater than or equal to 0. Noninteger values are truncated.
Remarks

If the value of either argument is outside of its constraints, COMBINA returns the #NUM! error value.

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

The following equation is used:

In the equation above, N is Number and M is Number_chosen.
Example
Copy the example data in the following table, and paste it in cell A1 of a new Excel worksheet. For formulas to show results, select them, press F2, and then press Enter. If you need to, you can adjust the column widths to see all the data.
Formula 
Description 
Result 
=COMBINA(4,3) 
Returns the number of combinations (with repetitions) for 4 and 3. 
20 
=COMBINA(10,3) 
Returns the number of combinations (with repetitions) for 10 and 3. 
220 