4 servings per day, 1 serving is 250 mL so total milk consumed in one day is 4 x 250 mL = 1000 mL
The carton contains 2 Liters of milk and there are 1,000 mL in one liter, so the carton contains 2,000 mL of milk. 2,000 mL/1,000 mL = 2 days of milk in the carton.
You can also think about it conceptually. If I drink 250 mL from a carton of milk that contains 2,000 mL, then there are 1,750 mL of milk left. Another 250 mL serving and the milk in the carton goes to 1,500 and so on until you have the 8th serving of milk on the second day of the milk carton.