A chemist mixes a 10%-acid solution with a 40%-acid solution to create 300 milliliters of a 25%-acid solution. How many milliliters of the…

Let $b$ be the milliliters of 40% solution, so $300 - b$ is the amount of 10% solution. The acid in the final mixture equals the acid from each part: $0.10(300 - b) + 0.40b = 0.25(300)$. That simplifies to $30 - 0.10b + 0.40b = 75$, so $0.30b = 45$ and $b = 150$, choice C. Sanity check: 25% is exactly halfway between 10% and 40%, so the two amounts must be equal. Choice A (100) would give a mix closer to 10%, choice B (200) a mix closer to 40%, and choice D (120) also fails to balance the acid, giving 22%.