What is the calculation for the combination index?

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The combination index is calculated using the formula:

C(n,r) = n! / [(n-r)! * r!]

Where: - n is the total number of items in the set - r is the number of items to choose from the set - ! denotes factorial, which means multiplying the numbers from n down to 1

For example, if you have a set of 5 items and want to choose 3 of them, the combination index would be:

C(5,3) = 5! / [(5-3)! * 3!] = 10

This means that there are 10 possible combinations of 3 items that can be chosen from a set of 5.