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 15% solution is Januet using in the mixture?

Let x be the liters of 15% solution, so the 35% solution contributes 35 − x liters. Equating the amount of glucose gives 0.15x + 0.35(35 − x) = 0.19(35), which simplifies to x = 28. Janet is using 28 liters of the 15% solution, choice C.

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