State the formula you can use for working out the probability of getting A or B?

P(A u B) = P(A) + P(B) - P(A n B), if mutually exclusive events then P(A n B) = 0 and so P (A u B) = P(A) + P(B)

P(A or B) = P(A) + P(B) - P(A and B), for some intuition behind this result think of a venn diagram with A and B now we see the probability of A or B should be related to the sum of P(A) and P(B) but notice we have counted the intersection of A and B on the venn diagram twice (once with P(A) and again when we count P(B)) therefore to cancel out this double counting we take away P(A and B).

For mutually exclusive events ( events that can not happen at the same time)

P( AUB) = P(A) + P(B)

For events that are not mutually exclusive ( where there is an intersection)

P( AUB) = P(A) + P(B) - P(AnB)

You have to subtract the intersection otherwise this is counted twice and you may get an answer bigger than 1

or = U

P(A or B) = P(A U B) = P(A) + P(B) – P(A ∩ B)

The probability that only one of the events A and B occur is equal to P(A) + P(B) - 2P(A,B). However, the probability that A, B or both events happen, is equal to P(A) + P(B) - P(A,B), which can also be represented as A ∪ B.

p(A) + p(B) if are mutually exclusive events.

Pr(A or B) = Pr(A) + Pr(B)