The constant of proportionality relates the independent variable to the dependent variable, such that the dependent variable is a constant multiple of the independent variable. In this problem, the independent variable is the number of students (let's denote this as x), and the dependent variable is the cost (let's denote this as y). The equation is:
y = kx, where k is the constant of proportionality
$1260 = k(84 students)
k = $1260/84 students = $15/student
$1050 = k(70 students)
k = $1050/70 students = $15/student
$1395= k(93 students)
k = $1395/93 students = $15/student
Thus, the constant of proportionality is 15. This represents the cost of ticket per student.