Janet is mixing a 15% glucose solution with a 35% glucose solution. This mixture produces 35 liters of a 19% glucose solution. How many liters of the 35% solution did Janet use?

Solution:

We need x liters of the 35% solution

So

x*35%+(35-x)*15%=35*19%

x=7.78

So 7.78 liters of the 35% solution is needed