I am not a “Math Nerd©” but even I know that what the item is doesn’t matter to answer this question. The question is about how to compare what the savings are per item bulk vs individual. It could be cars or bananas, what the item is is not important to the question. The question is asking for formula by example, not for someone to do their example math problem and return an itemized price sheet.

Returns and defects can effect savings per item, but the question does not include a clause that requires this calculation. Since the question does not mention it, it can be assumed OP was asking about savings before any defects or returns are made. This is not a net savings question, it is a gross savings question. Gross savings of course being the savings before any modifiers such as sales or import taxes (which are also not mentioned and therefore rightfully ignored by the person you attempted to “correct”), or in this case, defects and returns.