NUST NET Maths · Maths · Permutations, combinations and probability. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
When all outcomes are equally likely, the probability of an event is the number of favourable outcomes divided by the total number of outcomes. It always lies between 0 and 1, and an event and its complement add up to 1.
The addition rule is for "A or B". If the two events can happen together, the overlap must be subtracted once. The multiplication rule is for "A and B". For independent events simply multiply; when items are drawn without replacement, the second probability changes because one item has gone.
For "at least one", it is nearly always quicker to find the probability of none and subtract it from 1.
Adding P(A) and P(B) for "or" without subtracting the overlap. Kings or hearts is 4/52 + 13/52 − 1/52 = 16/52, because the king of hearts would otherwise be counted twice.
Answer: 8/15
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