Excel Function COMBINA

COMBINA(number, number_chosen)

Returns the number of combinations for a given number of items (with repetition).

number	The total number of items.
number_chosen	The number of items in each combination.

Remarks

* Added in Excel 2013.
* If "number" < 0, then #NUM is returned.
* If "number" < "number_chosen", then #NUM is returned.
* If "number_chosen" < 0, then #NUM is returned.
* If "number" is not an integer, it is truncated.
* If "number_chosen" is not an integer, it is truncated.
* If you want to return the number of combinations (without repetition) you can use the COMBIN function.
* For the Microsoft documentation refer to support.office.com

	A
1	=COMBINA(1,1) = 1 = a
2	=COMBINA(2,2) = 3 = ab, aa, bb
3	=COMBINA(3,2) = 6 = ab, bc, ac, aa, bb, cc
4	=COMBINA(4,2) = 10 = ab, bc, cd, dc, ad, bd, ad, aa, bb, cc, dd
5	=COMBINA(5,2) = 15
6	=COMBINA(6,2) = 21
7	=COMBINA(7,2) = 28
8	=COMBINA(8,2) = 36
9	=COMBINA(10,1)*COMBINA(10,1)*COMBINA(10,1) = 1000
10	=COMBINA("8","4") = 330
11	=COMBINA(1,2) = 1
12	=COMBINA("some text",2) = #VALUE!
13	=COMBINA(2,"some text") = #VALUE!