[oleg_kikin]

1) Chef has a list of 250 3-letter codes

2) He randomly picks 5 of them as "special"

3) He makes a list X of all unique letters in the codes of these 5 dishes

4) Guest picks a random dish. There are a few things that can happen:

   A) The code of that dish has a letter that's not on the list X. Ignore.

   B) All letters of that dish are on the list X. Two options:

      B1) The dish the guest picked is one of the 5 special.

      B2) The dish the guest picked is not one of the 5 special.

The first question is - how often does B2 happen?

The answer to the second question of that problem is obvious, you don't need to write any code.

[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.