Probability of A or B

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•  if A and B ARE mutually exclusive then the events cannot occur at the same time
• Then there is no intersection
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• Since they cannot both occur so in that case it's just P(A) + P(B)
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• For NON mutually exclusive events they can both occur at the same time
• so we have an intersection of the events A and B, so we still have P(A) + P(B)  but now see if we add A and B we are also adding the intersected part, so really adding a part of A and a Part of B twice  so to just get A or B we have to remove that part, so subtract off P(A and B)