I found this one strange because there is no stated relationship anywhere in in the problem between mol's A and B. We have no idea if they are reactants with each other, self-reacting, reacting with something else. Once you make the assumption that they react with each other, the logic of the key makes sense. I think that BR needs to add a reaction to this chart so that it is a more reasonable problem. Generally AAMC will have a reaction somewhere in the passage related to an experimental result such as gas chrom, IR, radiolabeling, etc. Or some text giving context to the concept that is being asked about.