Let x = small drinks
Let y = large drinks
3 small drinks and 2 large drinks contain 108 ounces of cola, this is:
3x + 2y = 108
A small drink contains a third as much cola as a large drink, this is:
x = 1/3y
Then, we solve the system of equations:

First, substitute x in equation 1:

And solve for y:

Next, substitute y = 36 in x:

Answer:
Small drinks: 12 ounces of cola
Large drinks: 36 ounces of cola