OFFERS, On June 1, Jack placed an ad in a local newspaper, to be run on the following Sunday, June 5, offering a reward of $100 to anyone who found his wallet. When his wallet had not been returned by June 12, he purchased another wallet and took steps to obtain duplicates of his drivers' license, credit cards, and other items the he had lost. He also, placed another ad in the same newspaper revoking his offer. The second ad was the same size as the original. On June 15, Sam, who had seen Jack's first ad in the paper, found Jack's wallet, returned it to Jack, and asked for the $100. Is Jack obligated to pay Sam the $100? Why or why not?