You and a group of friends are planning to visit a theme park, which charges $$60$ for admission, $$100$ for a two$-$day pass, and $$130$ for a three$-$day pass. Your friends are interested in spending a lot of time there, but they're worried about paying a lot of money. You explain the concept of marginal cost, which helps them see that the additional day is a good value. If there are $6$ people in your group, the group's marginal cost of switchinh from the two$-$day pass to the three$-$day pass is$?$