Muffin. Remember, to calculate marginal utility per dollar, divide the marginal utility by the price (MU / P). For the doughnut, MU / P = 30 / $2. For the muffin, MU / P = 20 / $1. The muffin has a marginal utility per dollar of 20, which is higher than the doughnut's MU per dollar of 15, making the muffin the better buy.