Play Time is a nursery school for pre-kindergarten children. The school has determined that the following biweekly revenues and costs occur at different levels of enrollment: Number of Students Total Revenue Total Costs Enrolled 10 $2,800 $1,500 15 4,200. 2,400 16 4,480 2,560 20 5,600 3,210 21 5,880 3,285 The marginal cost when the twenty-first student enrolls in the school is: