What is the linear function for the relation between the number of participants and cost if the cost for 20 students is $600 and $1,000 for 100 students?

(20,600) and (100,1000)

slope = m = (1000-600)/(100-20)
= 400/80 = 5
so
cost = 5 (number) + b
substitute in one point
600 = 5(20) + b
b = 600 - 100 = 500
so
cost = 5(number) + 500