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If you toss a coin, the probability of obtaining a head is ^{1}/_{2} and the probability of obtaining a tail is also ^{1}/_{2}.
P(head) + P(tail) = ^{1} / _{2} + ^{1} / _{2} = 1
If we choose a letter at random from the word 'SUMS', the probability of obtaining the letter 'S' is ^{2} / _{4}, the probability of obtaining the letter 'U' is ^{1} / _{4} and the probability of obtaining the letter 'M' is ^{1} / _{4}.
P(S) + P(U) + P(M) = ^{2} / _{4} + ^{1} / _{4}+ ^{1} / _{4} = 1
Remember that the sum of the probabilities of all possible outcomes is 1
The probability that I am late for work tomorrow is ^{2}/_{9}. What is the probability that I am not late for work?
There are two possible outcomes - being late and not being late. The sum of their probabilities must add up to 1, so the probability of not being late is 1 - ^{2}/_{9} = 7/9
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