The cost when p=2 is 27.54
The cost when p=3 is 41.31.
The difference between costs when p=2 and p=3 is 41.31-27.54=13.77
The cost when p=4 is 55.08
The difference of costs when p=4 and p=3 is 55.08-41.31=13.77
Thus, for each additional person the cost is 13.77.
So if the total cost is C and number of people is p, the equation for total cost is
C=13.77p