The first thing to consider is whether Jack makes an offer. It would be impressive to note the similarities with Carlill v Carbolic Smoke Ball Co  1 QB 256 but also perhaps to distinguish Carlill since no money is deposited in the bank as a show of sincerity.
One common error concerning Curtis is to think the postal rule applies. This is wrong—the postal rule only applies to acceptances.
Jack’s second advert might be too late to revoke the offer; this requires discussion of eg Errington v Errington  1 KB 290. And some consideration of how an offer to the whole world can be revoked, given the general need for communication. But in order to consider the rest of the question, you might assume it is arguable that the revocation could be effective, or else deal with the other competitors as part of the coda to the question.
David emphasises the possible requirement of communication. Edgar’s position might be linked to that in Dickinson v Dodds (1876) 2 ChD 463, but is possibly distinguishable if it is not clear that the information comes from a trustworthy source. Tony prompts discussion of whether you need to know of an offer in order to accept it (cf Gibbons v Proctor).