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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 are a finite number of equally likely outcomes, the probability of an event is given by

The '''probability''' of an event is a number between 0 and 1 representing the likelihood of the event's occurrence.  It may be expressed as a simple fraction, decimal fraction, or percentage. When there are a finite number of equally likely outcomes, the probability of an event is given by

<math>

<math>

</math>.

</math>.

For example, the probability of a tossed fair coin coming up heads is &frac12; = 0.5 = 50% because there is one favorable outcome (heads) out of 2 possible outcomes (heads and tails). Another common example is 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 = 0.20</math>, or 20%.

For example, the probability of a tossed fair coin coming up heads is &frac12; = 0.5 = 50% because there is one favorable outcome (heads) out of 2 possible outcomes (heads and tails). Another common example is 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%.

==Formal definition of probability==

==Formal definition of probability==