67. There are two alcohol solutions, solution A has a concentration of $75\%$, and solution B has a concentration of $15\%$. Now, we need to mix these two solutions to form a $50\%$ alcohol solution of 18 liters. We should take solution A $\qquad$ liters and solution B $\qquad$ liters.
Reference answer: $10.5 ; 7.5$