Two dietary drinks are used to supply protein, carbohydrates, and vitamin C. The first drink provides 2 units of protein, 5 units of carbohydrates, and 7 units of vitamin C in each liter. The second drink supplies 4 units of protein, 4 units of carbohydrates, and 6 units of vitamin C in each liter. An athlete requires 10 units of protein, 15 units of carbohydrates, and 20 units of vitamin C. If the first drink cost $8 per liter and the second costs $6 per liter, find the amount of each drink the athlete should consume to minimize the cost and still meet the minimum dietary requirements.

Solve this using graphical method.