If liquor is 80 proof, what percentage of ethanol does it contain?

When a liquor is labeled as "80 proof," this indicates its alcohol content. In the United States, proof is defined as twice the percentage of alcohol by volume (ABV). Therefore, to determine the percentage of ethanol in an 80 proof liquor, you would divide the proof by two.

In this case, 80 proof means that the liquor contains 40% ethanol. This understanding comes from the proof system, where a proof value of 100 corresponds to 50% alcohol by volume, and so forth. Consequently, 80 proof translates to 40% ABV, which means the correct answer is that an 80 proof liquor contains 40% ethanol.