nCr, also written as 'n choose r', is a mathematical notation used to denote the number of combinations of n items taken r at a time. This concept is essential for determining how many ways a subset can be chosen from a larger set without regard to the order of selection. The formula for nCr is given by $$nCr = \frac{n!}{r!(n-r)!}$$, where '!' represents the factorial operation. Understanding nCr helps in solving problems related to selections and arrangements in various fields.
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