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The grouping of a subset of a set of items where the order does not matter is called a combination. One such example is the UK's National Lotto where 6 numbers have to be chosen from the 59 numbers 1-59).

If there are n different items and a subset of r of them are chosen where the order of choosing does not matter then the number of combinations is given by:

nCr = n!/((n-r)!r!)

where n! means "n factorial" - the product of all numbers 1 × 2 × ... × n; 0! is defined to be 1.

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Where the order of selection does matter, it is called a permutation. One such example would be the order of the first three runners in a race.

If there are n different items and a subset of r of them are chosen where the order of choosing does matter, then the number of permutations is given by:

nPr = n!/(n-r)!

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