Problem: Bamang agrees to do 50 hours of research for Yanti at a price of $1000. When Bambang is finished, Yanti is very happy with the quality of the research and promises to give Bambang a bonus of $200. Later, Yanti gives Bambang $1000 but explains she will not give the bonus as she has changed her mind. Under the rules of consideration, can Bambang enforce Yanti's promise regarding the bonus for $200? In your answer, make sure to use relevant case law to achieve the full marks!