Probability
The probability of an event is a number between 0 (representing impossibility), and 1 (representing certainty) and may be expressed as a simple fraction, decimal fraction or percentage. When there is a finite number of equally likely outcomes then the probability of an event is given by <math>\frac{\mbox{number of favourable outcomes}}{\mbox{number of possible outcomes}}</math>.
For example, The probability of a tossed fair coin coming up heads is ½ = 0.5 = 50% because there is one favourable outcome (heads) out of 2 possible outcomes (heads and tails).
With a standard deck of 52 playing cards, there are 13 cards in each suit: spades, hearts, clubs and diamonds. After the deck is shuffled thoroughly, the probability that the top card on the deck is a heart is <math>13 / 52 = 1 / 4 = 0.25</math>, or 25%.