What is the formula for the combinations available in a set of 25 numbers, with the following restrictions?

This particular problem does not allow sets that contain any duplicate constituents, at all. So no sets that look like (A,A) or (B,B), or (Y,Y,Y...,Y), or (A,B,B), etc. Moreover, the sets that have the same numbers in different order all count as the same set. So, (A,B,C,D)=(C,B,A,D)=(D,A,B,C), etc. Those count as one set, not multiple sets. I know that 25! includes all possible combos, including duplicates and the same numbers in different order. But I can't remember the formula that eliminates the duplicates and the redundant sets. Thanks for the help.

What is the formula for the combinations available in a set of 25 numbers, with the following restrictions?
if you take the numbers one-by-one: 25! / ( (25-1)!*1! )





combination from n numbers taken by k: n! / ( (n-k)!*k! )





for example:


-25 numbers, taken by 7: 25! / (18!*7!)


-25 numbers, taken by 4: 25!/ (21!*4!)

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