The answer is maximizing utility.
When Marietta is making her decision she is trying to get the maximum utility for the money that she has.
This is an example of the Utility Maximizing Rule. (Keep in mind that utility is defined as the total amount of satisfaction a consumer obtains from consuming a product.) The utility maximizing rule explains how consumers decide to allocate their money so that the last dollar spent on each product purchased yields the same amount of extra (marginal) utility.
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