Final answer:
The marginal utility per dollar spent on good a is 2.
Step-by-step explanation:
To find the marginal utility per dollar spent on good a, we need to divide the marginal utility from good a by the cost of good a. In this case, the marginal utility from good a is 20 and the cost of good a is $10. So, the marginal utility per dollar spent on good a is 20/10 = 2.