Answer all questions in detail

1. What is a contract and its essential elements?
2. Sara decides to sell her car and puts a notice on the back windscreen of the car. The notice reads, 'This car is for sale, excellent condition, low mileage, bargain at PKR 20 lacs. Please phone 04234567891 or call in at 8 Kareem Street.' On Monday morning Tahir telephones Sara and offers to buy the car for PKR15 lacs. Sara refuses to accept his offer. Later that day Tahir changes his mind about the car and telephones Sara. She is out so he leaves a message on her answerphone saying, 'I agree to give you the full asking price of PKR20 lacs for your car and will be round to collect it tomorrow'. On Tuesday morning, Aslam sees Sara's car and writes a letter of acceptance to Sara enclosing a cheque for PKR20,lacs. He immediately puts the letter in the post box. On Tuesday afternoon Farah calls at 8 Kareem Street but Sara is out, so she puts a note through the letterbox stating, 'I have had a good look at your carit is just what I wantI enclose a cheque for the full asking price of PKR20,lacs'.

Explain to Tahir, Aslam, and Farah if they have a valid contract to purchase Sara's car, giving full reasons.