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

This problem is an another case which makes use of the concept of ratio and proportion. The ratio $2.52 per 0.5 gal should be proportional to the price of 4 gallons of milk. Let x be the price,
                      ($2.52 / 0.5 gallon) = (x / 4 gallons)
The value of x is 20.16. Hence, the price of 4 gallons of organic milk is $20.16.