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The number of permutations of n different things taken r at a time, where repetition is not allowed is 
, where 
The number of permutations of n different things, taken r at a time, where repetition is allowed, is nr.
The number of permutations of n objects taken all at a time, where p1 objects are of the first kind, p2 objects are of the second kind, … , pk objects are of the k^th kind and rest, if any, are all different is 
The number of combinations of n different things taken r at a time is 
,
