For a field trip, a bus company charges a flat fee plus an additional fee per student. For 25 students, the total cost is 135.50. If there are 30 students, the total cost is 145.

Question

Write an equation that represents the total cost y of the field trip for x students.

I don't understand how to find the flat fee.

Answer 1

The answer comes from the derivation of a straight line model for the cost vs students. Note that 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 thus verifying the math model works for the data points provided.