Susan wishes to buy gasoline and have her car washed. she finds that if she buys 9 gallons of gasoline at $2 per gallon, the car wash costs $1.50, 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

The marginal cost is how much must be added from the ninth to the tenth gallons. For the 9 gallons and car wash, she would have paid ($2/gallon) * (9 gallons) + ($1.50/car wash) = $19.50

For the 10 gallons and car wash, she would pay ($2/gallon) * (10 gallons) + ($0/car wash) = $20.00

Therefore her marginal cost is $20.00 - $19.50 = $0.50