the $25 is a relevant cost for Joe, whereas your $30 is a sunk cost-and hence an irrelevant one for you 9. For a seven-minute call the two phone sy stems charge exactly the same amount, 70 cents. But at that point under the new plan, the marginal cost is only 2 cents per minute, compared to 10 cents per minute under the current plan. And since the benefit of talking add itional minutes is the same under the two plans, Tom will make longer calls under the new plan 10. In University A, everybody will keep eating until the benefit from eating an extra pound of food is equal to $O, since that is the extra cost to them for each extra pound they eat. In University B, the cost of eating an extra pound of food is $2,so will stop eating when the benefit of eating an extra pound falls to $2. Food consumption will thus be higher at University Athe $25 is a relevant cost for Joe, whereas your $30 is a sunk cost—and hence an irrelevant one for you. 9. For a seven-minute call the two phone systems charge exactly the same amount, 70 cents. But at that point under the new plan, the marginal cost is only 2 cents per minute, compared to 10 cents per minute under the current plan. And since the benefit of talking additional minutes is the same under the two plans, Tom will make longer calls under the new plan. 10. In University A, everybody will keep eating until the benefit from eating an extra pound of food is equal to $0, since that is the extra cost to them for each extra pound of food they eat. In University B, the cost of eating an extra pound of food is $2, so people will stop eating when the benefit of eating an extra pound falls to $2. Food consumption will thus be higher at University A