On 10 January Martha wrote a letter to Jimmy offering to sell him her laptop computer for $1,000.

On 11 January Jimmy received the letter and he replied by letter accepting the offer.

On 12 January Jimmy posted the letter of acceptance first thing. On the afternoon of 12 January, Jimmy changed his mind and decided that he didn't want to buy the laptop. He then sent a fax to Martha saying: "Changed my mind, ignore letter of 12 January". Martha received the fax.

On 13 January Martha sold the laptop to Robert.

On 14 January Jimmy's letter of acceptance arrived.

On 15 January Jimmy phoned Martha to say that he had changed his mind again and that he now wants to buy the laptop.

Is Martha in breach of a contract with Jimmy? Refer to general law principles in your answer. Use your own words at all times.