Pam intended to sell her bicycle and she drafted a poster and hanged it on campus. The poster read as follows:

'A nearly brand new mountain bicycle in good condition for 60.' There she left a picture of the bicycle and her contact as well.

Jack called Pam the next day on Tuesday, showed his interests to acquire the bicycle. However, Jack was only willing to offer 30 for the bicycle. Pam insisted that she would not sell it unless Jack pay her the full price of 60.

On Thursday, Jack received a letter from Pam which stated that she would be happy to sell the bicycle for45, but Jack has to reply to her before noon on Friday. Pam immediately replied to Pam to accept her offer and he posted the letter the next day.

On Thursday evening, Pam changed her mind and would like to keep her bicycle and so she wrote a letter to Jack - stating she would not sell her bicycle therefore. However, this letter did not reach Jack until Saturday. And Jack's letter were received by Pam on the following Monday.

Required.

a)Advise Jack his position regarding the sale and purchase of the bicycle

b)Explain in what circumstance a unilateral contract can occur.