Combinations
Combinations are of two types:
1 In which repetition is allowed
2. In which repetion is not allowed
Combinations with repetition
(n+r-1)!/r!(n-1)!
Where n is the number of things to choose from and you choose r of them
Combinations without repetition
A simple example of this would be lottery numbers.
If we assume that the order does matter, i.e., permutations, alter it so the order does not matter.
Formula: n!/r!(n-r)!
Where n is the number of things to choose from, and you choose r of them
It is also known as "n choose r" or binomial coefficient.
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