Summary of permutations, combinations and probability

Last updated: 18 Jul 2007

You should remember the following

Permutations

Permutations are where order does matter

`text{Permutations} = (n!)/(n-r)!`

Combinations

Combinations are where order does not matter

`text{Combinations} = (n!)/(r!(n-r)!)`

Basic rule of probability

When each possibility is equally likely.

`P(text{outcome}) = text{number of ways it can happen} / text{total number of outcomes}`

'And' probabilities

The probability of two independent outcomes both occurring can be found by multiplying their individual probabilities together.

`P(text{A and B}) = P(A) xx P(B)`

'Or' probabilities

The probability that either of two exclusive outcomes will occur can be found by adding their individual probabilities together.

`P(text{A or B}) = P(A) + P(B)`