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 Let E the sample space of a random experiment and S the set of random events, then the probability is a function:

which verifies:


Example: The result of a poll, done to Bachillerato students in a school, says that 40 % of them read a newspaper and 30 % read an information magazine. We know that 20 % read both of them, too. With these data, which is the probability that a randomly chosen student reads a newspaper or a magazine?
Let N = {read newspaper} and M = {read magazine}. The probabilities are:
P(N) = 0,4      P(M) = 0,3    P(N∩M) = 0,2
P(NUM) = P(N) + P(M) – P(N∩M)= 0,4 + 0,3 – 0,2 = 0,5