Organic milk costs $2.52 per half gallon. at this rate, how can the price of 4 gallons of organic milk be determined?

The problem above uses the concept of ratio and proportion. The ratio between the cost and amount of the organic milk should always be equal. The proportion between two cases is shown below, 
                        ($2.52 / 0.5 gallon) = (x / 4 gallons)   
Solving for x gives x = 20.16. Therefore,  4 gallons of organic milk costs $20.16.