Despite solving her puzzle, Joyce cursed Ketaki and Roshni's CAT papers such that their systems crashed as soon as they submitted their scores (Joyce's Wrath continues...). They decided to fight back and exact some revenge on Joyce while Prometric was busy rescheduling their exams. They thought their scores would be safe with Joyce out of the way. They collected 21 empty bottles akin to the ones in which they had trapped Jayshri and Vandana. They completely filled 7 of those bottles with a blood-red liquid, half-filled 7 bottles with the same liquid and left the remaining 7 bottles empty. Then Roshni beckoned Joyce into the room and challenged her. Said Roshni, "This has gone far enough. Now here's a challenge! Here are 21 bottles - 7 completely filled, 7 half-filled and 7 empty. They're for you and your sisters - Vandana and Jayshri. How will you divide the liquid and the bottles equally among yourselves? You cannot use any measuring devices for this. If you manage to answer this, we'll set your sisters free. If you cannot, you enter one of the empty bottles and get trapped! Dare to take up this challenge?" Joyce accepted this for the sake of her sisters. Help her crack this puzzle. Provide a detailed explanation along with your answer.