At a sporting event concession stand, a small bottle of water costs $2.50 and a large bottle costs $3.00. The concession stand employees collected a total of $357 from water bottle sales. They sold twice as many large bottles as small bottles. Let s represent the number of small bottles sold and let l represent the number of large bottles sold. How many of each size water bottle did they sell?

First we have to setup the system of equations for the given case.
Small bottles=s
Large bottles=l
Equations are:
l=2s Equation 1
2.5s+3l=357 Equation 2
Substitute value of l in equation 2.
2.5s+3(2s)=357
2.5s+6s=357
8.5s=357
s=42
Now substitute value of s in equation 1.
l=2(42)
l=84

So they sold 42 small bottles and 84 large bottles of water.
Total bottles= 126