The main thing you need to know for this question is this conversion:
1 L = 1000 mL , or 1 liter is equal to 1000 milliliters.
So, let's make the entire problem into milliliters:
The carton of milk has 2,500 mL, and Zoe drinks 500 mL a day (250 twice a day).
To find the number of days the bottle will last, you divide the total amount of milk by the amount she drinks per day:
2,500 / 500 = 5 days
