Contract or No Contract?

Applying the theory of contract law, discuss if these 4 situations have formed an enforceable contract.

3. Susan: I will sell you my laptop for $500. Jenny: No, I dont want it for $500. Jenny: But Ill give you $400 for it. Susan: No way, it's like brand new! Jenny: Ok, Ill take it for $500.

Do Susan and Jenny have a contract or not? Why?

4. Lynn's cat is missing in her neighborhood. She posts a sign with a picture that states: "$50 reward for finding Lucy". Lynn asks her friend James to walk around with her and look for Lucy. He agrees. After about 40 minutes, a stranger approaches Lynn and James with Lucy the cat and asks them if they know who she belongs to. Who does Lynn owe the reward money to and why?