John has been in the habit of mowing Willa's lawn each week for $20. John's opportunity cost is $15, and Willa would be willing to pay $25 to have her lawn mowed. What is the maximum tax the government can impose on lawn mowing without discouraging John and Willa from continuing their mutually beneficial arrangement?

If the tax is less than $10, there will exist a price at which both John and Willa will still benefit from the lawn-mowing arrangement. If the tax is $10, a price can be set which will leave John and Willa neither better off nor worse off from the lawn-mowing arrangement. If the tax is greater than $10, all possible prices will leave at least one of the parties worse off from the lawn-mowing arrangement.