the change in cost of 145-135.50 = 9.50 for an increase of students from 25 to 30. The ratio of change of cost to change of students = 9.50/5 = 1.9 and becomes the slope coefficient "m" in the formula y= mx + b. y = the total cost, mx becomes the variable cost and b becomes the fixed cost. To find b, use the data point given where the total cost = $135.50 when the students = 25, or 135.50 = 1.9*25 +b. Solving for b yields $ 88. Note that the other data point is where the total cost is $145 for students = 30. Using the new total cost equation shows that 30*1.9+88= 145
