Maria has a budget of $30 and is considering purchasing two goods: water and good X. Water costs $3 per unit, and good X costs $6 per unit. If Maria derives a marginal utility of 9 from the last unit of water and 12 from the last unit of good X, how should she allocate her budget to maximize her total utility?

To maximize her total utility, Maria should compare the marginal utility she gets from each dollar spent on water and good X. For water, the marginal utility per dollar is 9 divided by 3, which equals 3. For good X, the marginal utility per dollar is 12 divided by 6, which equals 2. Since the marginal utility per dollar for water is higher, Maria should spend more on water. Therefore, she should buy as much water as her budget allows before purchasing good X, ensuring she gets the most satisfaction from her money.