We first have to understand West's argument and its flaws. West maintains that Haynes is the worst quality control inspector because over half of the appliances that were returned due to defects were inspected by Haynes. The problem with the argument is that Haynes may be responsible for the majority of the inspections. Imagine Haynes is so good at his job that he does 99% of the inspections. If this were the case, it would be no surprise that over 50% of the appliances that were returned would have been inspected by Haynes. So the initial argument has a necessary assumption, or a presupposition, that Haynes does not conduct most of the inspections.
Young is denying this presupposition by suggesting that Haynes is, in fact, responsible for the vast majority of the inspections.
The takeaway here is that it is important to be critical of the arguments and how they function in order to fully understand the structure.