Susan wishes to buy gasoline and have her car washed. She finds that if she buys 9 gallons of gasoline at $1.50 per gallon, the car wash costs $1, but if she buys 10 gallons of gasoline, the car wash is free. For Susan, the marginal cost of the tenth gallon of gasoline is

Question

Quizplus · Accepted Answer

The cost of 9 gallons of gasoline is (9 x $1.50) = $13.50. With the car wash added, the total cost is $14.50. 
The cost of 10 gallons of gasoline is (10 x $1.50) = $15. With the free car wash added, the total cost is $15. 
Therefore, the cost of the tenth gallon of gasoline is ($15 - $14.50) = $0.50. The marginal cost is the additional cost of producing one more unit, which in this case is the cost of the tenth gallon.

Susan Wishes to Buy Gasoline and Have Her Car Washed