A farmer sells whole milk from his cows at 80 cents a liter. Skim milk, on the other hand, sells it at 50 cents a liter. A customer asks for 50 liters of semi-skimmed milk that is obtained with the mixture of the other two and is sold at 60 cents.
How many liters of whole milk did he need to serve his client?
If we call E the liters of whole milk in the mixture and D the liters of skim milk, we have to: E + D = 50 liters.
We also know that E × 80 € + D × 50 € = 50 × 60 € so that we have a system of two equations with two unknowns and clearing that we obtain that E = 50/3 liters of whole milk.