[theycallhimtom]

When you say the guest picks a random dish, does he pick a random 3 letter code, or a random one of the 250 dishes?

You want the probability that B2 happens given that B happened right? (prob(B2) / prob(B) not prob(B2) / prob (A or B))

Thanks

[oleg_kikin]

No, guest picks one of the 250 dishes, and the probability of B2 given that either A or B happen.