Mary, a frail but mentally sound 87-year old woman, is in the front garden of her old weatherboard home. Duncan, a big man who was heavily tattooed and wearing leathers, parks his motorbike outside her garden and strikes up a conversation with Mary. He told Mary that: "I am a licensed painter and I could paint the exterior of your house for a good price - $8000". Mary obviously felt intimidated by Duncan and hastily agreed. The next day Duncan arrived and over the next two days painted her house. During this time Mary discovered from her daughter's inquiries that had Mary received competitive quotes for the painting work, the going rate for painting her house was about $4000. Her daughter also discovered that Duncan's registration as a licensed painter had lapsed two weeks ago because he had forgotten to pay the renewal fee. The job is now complete, and Duncan has asked Mary for $8000. Is Mary contractually obligated to pay Duncan the $8000? If yes, why? If not, why? What arguments can she make? What arguments can Duncan make?