For either of two events happening, add their probabilities and then take back the overlap, P(A∪B) = P(A) + P(B) − P(A∩B)
Module goal. Treat events like sets you can add and subtract, building the addition formula and the language of mutually exclusive and complementary events
eastmath.com · 33.3 Operations and Relationships Between Events · 33.3.3 The Addition Formula for Probability