Answer: c. They would do better charging $15 than $10.
Step-by-step explanation:
It is given that Max and Maddy total cost of car parking remains the same, that is it does not matter whether they have seven or five cars parked on their lawn. So, since total cost is constant, Max and Maddy's decision would depend on total revenue.

Total revenue from charging, $10 is $70,
Total revenue from charging $15 is $75.
Since, total cost is constant, Max and Maddy will choose to charge $15 since it is giving them $5 more.