Check the picture below, so, that'd be the square inscribed in the circle.
so... hmm the diagonals for the square are the diameter of the circle, and keep in mind that the radius of a circle is half the diameter, so let's find the diameter.
that means the radius r = 5.
now, what's the center? well, the Midpoint of the diagonals, is really the center of the circle, let's check,
so, now we know the center coordinates and the radius, let's plug them in,
Okay so 6.25 a student then it goes to 4.50 for 15 more students at 70 students its 358.75 now it drops another 1.75 so it would be 368.75 for 80 students so the equation is 4.48 x 80 = 368.75. enjoy :D
You may have $26 I not a genius but this is easy