The number of permutations of n distinct objects is n! = n(n − 1)(n −
2)···(3)(2)(1) The number of permutations of n objects, a of which are
indistinguishable, b of which are indistinguishable, etc., is
The number of permutations of n objects taken r at a time is
The number of combinations of n objects taken r at a time is
Probability
If the probability that an event will occur is a, then the probability that
the event will not occur is 1 − a.
If the probability that one event will occur is a and the independent
probability that another event will occur is b, then the probability that
both events will occur is ab.