On Tuesday, September 10, John delivered a written offer to sell a house to Mary. The offer stated that "This offer is to be left over until Thursday, 10 o'clock, September 12." On Wednesday, John sold the house to Jill. Lance found out about the sale to Jill and told Mary. At 8 a.m. Thursday, Lance acting as the agent of Mary, handed John a letter of acceptance. John had also posted an offer to sell his car to Ken . On the 1st September, John posted a revocation of his offer. The offer from John was received by Ken on the 4th September and a letter of acceptance was sent the same day. On the 10th September, John's letter of revocation reached Ken. Both Mary and Ken are contemplating suing John and John has come to you for advice. Advise John