If the market price is $25 in a perfectly competitive market, the marginal revenue from selling the fifth unit is

In a high school with 3000 students, counselors are putting together a math tutoring program for the neighboring middle school. they plan to invite high school students with high average test scores to tutor middle school students, and they would like to offer gift cards to the student tutors. the counselors have a spreadsheet showing each student’s average test scores, and in order to find out how many gift cards they should purchase, the counselors need to count how many students would qualify to become tutors. which type of function would best the counselors determine the number of eligible students?