33.4.4 The Total Probability Formula
Core ideaA ball might come from box one, two, or three, so weight each box by its share and add it all up, P(B) = ΣP(Aᵢ)P(B|Aᵢ)
Module goal. Tell apart "they don't affect each other" from "knowing one, now look at the other," and master the multiplication, conditional, and total probability rules