(a) On 1 February 2020, Jane wrote to John offering to sell to John her yacht, Thunder, for $500,000. The letter expressly stated that if John did not reply by 10 February 2020, he would be deemed to have accepted the offer. On 9 February 2020, Jane sent a letter to John revoking her offer. On 10 February 2020, John posted a letter to Jane accepting Jane's offer. Jane's letter of revocation reached John on 11 February 2020. John's letter of acceptance reached Jane on 12 February 2020. John wishes to know whether there is a valid contract between him and Jane for the sale and purchase of the yacht. Required: Advise John under the Law of Contract. (15marks)