Probability
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The probability of an event is typically expressed as the percentage of ways it might happen, out of a number of possible events. The chance of a tossed coin coming up heads is 50%, because there is 1 way it can happen, out of 2 possible ways. 1 / 2 = 0.5 or 50 percent.
With a standard deck of 52 playing cards, there are 13 cards in each suit: spades, hearts, clubs and diamonds. After you shuffle the cards thoroughly, what is the probability that the top card on the deck is a heart? 13 / 52 = 1 / 4 = 0.25 or 25 percent.
Probability is represented as a number between 0 (representing impossibility), and 1 (representing certainty). Sometimes percentages are used, with numbers from 0% to 100%.