Rule: The probability of an event A is calculated by summing the probabilities of the sample points in the sample space for A.
Classical definition of probability. If an experiment may lead to one and only one of n different equal outcomes and m is a number of favorable outcomes for occurrence of event A then the probability of event A can be calculated by formula
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In other words, the probability of an event A is calculated by summing the probabilities of the sample points in the sample space for A.
Example Take the sample space S for four tosses of a coin to be the 16 possible outcomes in the form HTHH. Then “exactly 2 heads” is an event. Call this event A. The event A expressed as a subset of outcomes is