Let G, E1 , E2 , E denote the followign events: G : Original signal is green E1 : A receives the signal correctly E2 : B receives the signal correctly E : B receives the green signal We have E = GE1E2 ∪ GE1′E2′ ∪ G′E1′E2′ ∪ G′E1′E2′ ⇒ P(E) = P(GE1E2) + P(GE1′E2′) + P(G′E1E2′) + P(G′E1′E2′)