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Permutations. If you had a 4 digit number and wrote that number into 4 blank boxes (one at a time, reducing the selections left by not re-using the same digit) how many way can it be written.
ie the number is 2134. We can write combinations or permutations of this as:
Any of the 4 digits x any of the 3 digits left x any of the 2 digits left x any of the 1 digit left = 4 x 3 x 2 x 1 = 24 ways of writing it.
It really doesn't matter what the digits are they could be any as the number of ways of writing permutations of it is the same.
In maths it's written 4!
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