"This whole idea of marginal utility is stupid. I mean, after all, who goes into a store and calculates the marginal utility from something before

"This whole idea of marginal utility is stupid. I mean, after all, who goes into a store and calculates the marginal utility from something before deciding to buy it? I just look at something, look at the price, think about how much money I’ve got in my pocket, and then decide whether or not to buy the thing. No one I know calculates marginal utilities when they go shopping, so why do I have to learn this stuff?”

Question: How would you respond to your classmate? (l disagree)