The table shows the total cost and the total individual benefit of having park rangers in a city park. Assume that the park has 1,000 visitors each day and that everyone's preferences are the same. Number of park rangers Total cost Total individual benefit 0 $0 $0 1 30,000 50 2 60,000 90 3 90,000 120 4 120,000 140 The marginal social benefit of the fourth ranger is $_____