In a salad dressing, the ratio of oil to vinegar is $3: 1$. To make the dressing more acidic, the amount of vinegar is doubled. What is the new ratio of oil to vinegar?
(A) $3: 2$
(B) $6: 1$
(C) $1: 3$
(D) $2: 3$
(E) $4: 3$
Suppose the salad dressing initially contains $300 \mathrm{~mL}$ of oil.

Since the ratio of oil to vinegar is $3: 1$, then the salad dressing initially contains one-third as much vinegar as oil, or $\frac{1}{3} \times 300 \mathrm{~mL}=100 \mathrm{~mL}$ of vinegar (note that $300: 100=3: 1$ ).

If the volume of vinegar is doubled, the new volume of vinegar is $2 \times 100 \mathrm{~mL}=200 \mathrm{~mL}$, and so the new ratio of oil to vinegar is $300: 200=3: 2$.

Note: We chose to begin with $300 \mathrm{~mL}$ of oil, but performing the above calculations with any starting volume of oil will give the same $3: 2$ ratio.

ANSWER: (A)