*n! = n Ã— (n â€“ 1) !*

*The number of permutations of n different things, taken r at a time, where repetition is allowed, is n*^{r}

*The number of permutations of n objects taken all at a time, where p*_{1} objects are of first kind, p_{2} objects are of the second kind, …, p_{k} objects are of the k^{th} kind and rest, if any, are all different is \( \frac{n!}{{p_1}! {p_2}! … {p_k}!} \)

*The number of permutations of n different things taken r at a time, where repetition is not allowed, is denoted by *^{n}C_{r} and is given by \( {^n}C_r = \frac{n!}{r! (n-r)!}\), where 0 â‰¤ r â‰¤ n.