Probability

$P(A) = \dfrac{same events to choose}{total events}$

Example case:
You have 9 red marbles, 2 blue marbles, 3 green.
Probability to choose any non-blue marble?
Answer:
P(A) = (14 total - 2 blue) / 14 total = 6/7

Permutations

N! - permutations formula for N cases.

Number of permutations of putting N people in R seats (n choose r):

$nPr = \dfrac{N!}{(N-R)!}$

0! = 1

Example case:
Possible 3 word letters from 26 english alphabet?
Answer:
26P3 = 26!/(26-3)! OR just 26 * 25 * 24

Combinations

Difference between combinations and permutations:

ABC, BCA, ACB, BAC - different permutations, but ONE combination

FBC, FCB, CBF, BFC - second combination

Number of combinations:

$nCr = \dfrac{N!}{R!(N-R)!}$