If cupcakes are tradeable for brownies then I'd distribute both evenly to start and allow people to trade at prices that seemed fair to them, but I assume that's not what you're talking about. And yeah, it's primarily an intuition, and one that I'm genuinely quite surprised to find isn't universal, but I'd probably try to justify it in terms of diminishing returns, that two people with 3 cupcakes each have a higher overall happiness than one person with 2 and one with 4.
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