Question

8. You find the probability P(A or B) by using the equation PA or B) = P(A) + P(B) - P(A and B).

Describe why it is necessary to subtract PA and B) when the events A and B are overlapping.
Then describe why it is not necessary to subtract P(A and B) when the events A and B are disjoint.

Answer 1

Explanation:

When you add P(A) and P(B), the overlap P(A and B) is counted twice. So you must subtract it to get the probability P(A or B).

If A and B are disjoint, then P(A and B) = 0, so it isn't necessary to subtract.