 The flash is timed so that it arrives at Bob just at the moment that Bob meets Carol, so Carol synchronises her clock to read formula_28.Also, when Bob and Carol meet, they both simultaneously send flashes to Alice, which are received simultaneously by Alice.Considering, first, Bob's flash, sent at time formula_27, it must be received by Alice at time formula_30, using the fact that the "k"-factor from Alice to Bob is the same as the "k"-factor from Bob to Alice.