A 12g mixture is 40% vinegar by weight. How many grams of oil must be added so the new mixture is 25% vinegar?

The vinegar weight stays at $(0.40)(12) = 4.8$g. Let $x$ be grams of oil added; the new mixture weighs $12 + x$ grams. Setting $4.8/(12 + x) = 0.25$ gives $4.8 = 0.25(12 + x) = 3 + 0.25x$. So $1.8 = 0.25x$, and $x = 7.2$g. Option B is the vinegar weight, not the answer.