Yasmin offers to landscape Bert's property for $2,000. Two days after making the offer, Yasmin changes her mind and mails Bert a letter revoking her offer. The next day, Bert, who has not received Yasmin's letter, calls Yasmin and accepts her offer. Does a contract exist?

Yes, a contract exists. Yasmin made a valid offer to Bert, and Bert accepted the offer before he was notified that it was revoked. In order for a revocation to be valid, the offeree (Bert) must be notified about it. If the offeree has accepted the offer before he/she is notified about the revocation, the acceptance is valid and the revocation is not valid.