Louisa needs 3 liters of lemonade and pouch for a picnic. she has 1,800 milliliters of lemonade. how much punch does she need?

There are 1,000 milliliters in 1 liter. To find the amount of liters of lemonade that she already has you must divide 1,800 by 1,000 which gives you 1.8 liters. To find the amount of punch (in liters) that she needs you must subtact 1.8 from 3 which gives you 1.2 liters of punch. To find the amount of punch in milliliters, multiply 1.2 by 1,000 which gives you 1,200 milliliters of punch.
Hope this helps!