Question 1196283
Let x = the number of liters of 30% vinegar as solution

Expressing the problem into equation:

0.15(4) + 0.3x = 0.2(4 + x)
0.6 + 0.3x = .8 + .2x

Combine like terms:
0.3x - 0.2x = 0.8 - 0.6
0.1x = 0.2
  x = 0.22/0.1
  x = 2

Answer:

The mixture required 2 liters of the 30% solution.