The problem is about friends who want to cleverly divide the barrels among themselves

The girls have 21 barrels in total. This means that each friend should get 21 ÷ 3 = 7 containers. If wine could be poured, then seven full and seven half-full barrels would make 7 + 7 × 0.5 = 7 + 3.5 = 10.5 kegs of wine. This means that each friend should get 10.5 ÷ 3 = 3.5 kegs of wine.

Let's compose this amount of liquid from seven full and seven half full containers that are available:

- the first friend will receive three full barrels and one half full;
- the second will also have three full containers and one half full;
- the third will get one last full barrel and the remaining five half full, for a total of 5 × 0.5 = 2.5 kegs of wine.

Now let's determine how many empty containers each friend will have. The first and the second already have four barrels of wine. To the required seven pieces, they need to add three empty ones. The remaining barrel without wine must be given to a third friend, who already has six containers.

Here's what happens: the first and second girls will have three full, one half full and three empty barrels, the third - one full, five half full and one empty.