"A scientist found the elixir of life. Unfortunatelly, he didn't record how to make it. All he knows is that there are 86 liters containing just 3.39 liters of the potion and that he mixed together a strong solution with 15% potion and a weak solution with 2% potion with water. His pet frog tells him that there are 20 liters more water in the mix than the other two solutions combined." We have to put this in equation; I think linear equation.