ABSTRACT
To tell whether a permutation or combination is involved, remember that the order of selection is important in permutations. The order of selection is immaterial for combinations. If we consider the letters u, v, and w, we have the following two-letter permutations: uv, uw, vw, vu, wu, and wv. There are, however, only three combinations of pairs of letters. We see that uv and vu, uw and wu, and vw and wv represent the pairs of permutations possible for each two-letter combination. From this example one should realize that the number of permutations of n objects taken r at a time is equal to the number of combinations of these n objects taken r at a time multiplied by the number of permutations of the r objects taken r at a time, which is r!. Thus
If the probabilities of simple events are known, the probabilities of compound (i.e., nonsimple) and related events may be computed in terms of the simple event probabilities. For clarity, certain terms, some of which have been defined previously, associated with such compound and related events are defined as follows (A and B are considered to be arbitrary events of an experiment):
1. If A is an event of an experiment, then the event in which A does not occur, denoted by A, is called the complementary event of A, or simply the complement of A.
