Probability of an event = # of ways an event can happen divided by # of ways anything can happen.
Example: Bag contains 3 green marbles, 2 red marbles, and 9 blue marbles.
Probability of picking a green marble = 3 (how many green marbles) / 14 (how many marbles all together).
Example: Deck of cards, normal 52 cards.
Probability of picking an ace = 4 (# of aces) / 52 (# of cards).
Example: Deck of cards, 54 card (52 + 2 jokers)
Probability of picking an ace = 4 (# of aces) / 54 (# of cards).