Anna is making a traditional bowl for her grandmother's birthday party. 9UP soda, two different liqueurs X and Y, and Possu liquor are used for bool, with alcohol concentrations of 0, 18, 18, and 37 percent and sugar concentrations of 24, 18, 17, and 3 percent (both percent per volume unit). Bool's traditions also include that there is 4 times more lime than liqueur Y. The finished bool should be 8.1 liters, and the alcohol content should be 7.79% by volume and the sugar content should be 21% by volume. How many liters of Possu liquor are needed to make bool?