Alan wants to sell his BMW X5 2020 model car and advertises it on 2 April in the local newspaper at $105,000 giving his telephone number. On the same day, Bob sees the advertisement and rings Alan, and makes an appointment to see the car. Bob likes the car but cannot agree on a price with Alan, his highest offer being $90,000 while Alan insists on $100,000. On 10 April, Bob receives a letter from Alan offering him the car for $95,000. On 14 April, Bob posted a letter to Alan, accepting his (Alan's) offer to sell the car at $95,000. Alan received the letter of acceptance on 16 April. However, on 15 April, Alan sold the car to his son-in-law Clever for $93,000. Therefore, Alan sent a reply to Bob on 17 April, saying "You are too late. I have sold the car to my son-in-law." Bob believes that a contract has been formed between Alan and Bob and he is entitled to enforce the contract. Bob seeks your advice if his understanding is correct. Advise Bob and justify your response, citing case authority whenever relevant. When answering this question, fully consider the relevant principles, rules and propositions in the Law of Contract. Do not consider any matters that might arise in the circumstances under other relevant legislation.