*Fundamental principle of counting If an event can occur in m different ways, following which another event can occur in n different ways, then the total number of occurrence of the events in the given order is m × n.*

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

*n! = 1 × 2 × 3 × …×n*

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*n! = n × (n – 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 ≤ r ≤ n.*