A alcohol solution has 35\% alcohol. Another solution has 25\% alcohol. How many liters of the second solution must be added to 18 liters of the first solution to make a solution of 30\% alcohol?

Let x liters of 25% solution be added. Total alcohol = 35% of 18 + 25% of x = 6.3 + 0.25x. Total volume = 18 + x. This should equal 30% of (18 + x): 6.3 + 0.25x = 0.30(18 + x) = 5.4 + 0.30x. Solving: 6.3 - 5.4 = 0.30x - 0.25x, so 0.9 = 0.05x, giving x = 18. You can also use allegation: (35-30):(30-25) = 5:5 = 1:1, so equal quantities.