DU LLB Entrance: Contract Law - A Study Experience to Remember

I still remember this one question that got me stuck during mock test - "Mukesh and Rohan entered into a contract for sale of goods of Rs 1,00,000. Rohan delivered goods worth Rs 75,000. Mukesh refused to pay, on the ground that the goods delivered were not as per the contract. Rohan sued Mukesh for breach of contract. Decide whether Mukesh was liable." I was blank on whether to apply Section 51 or 73 of the Contract Act. But then I remembered - it's a question of implied undertaking under Section 2(h) of the Sale of Goods Act. Got it right, but lost 4 marks for not applying the correct section. Scored 65/70. Need to work on my commercial laws, still struggling with sections 19, 20, and 21.