Factorial, Permutation and Combination

n! -> n factorial

0! = 1 

   nPr -> n permutation r 

nPn = n!

n ≥ r

nPr means the number of methods 

to choose r different elements among n different elements 

with order.

nCr -> n combination r

      n ≥ r     

nCr means the number of methods
to choose r different elements among n different elements
without order.

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Factorial

Permutation

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Combination