Final answer:
To calculate the proof of a drink with 20% ABV, simply multiply the ABV by two, resulting in a 40-proof beverage.
Step-by-step explanation:
If the alcohol by volume (ABV) of a beverage is 20%, the proof of the drink can be calculated by multiplying the ABV by a factor of two. In the United States, proof is exactly twice the percentage of ABV. Therefore, a beverage with 20% ABV will have a proof of 40 proofs.
The term proof originated in the UK, and while it is no longer used in the UK in this context, it remains a measure used in the United States to quantify the alcohol content in spirits and other alcoholic beverages. The proof of an alcoholic drink is twice the alcohol by volume (ABV). So, if the ABV of a beverage is 20%, the proof would be 40. For example, typical vodka is sold as '80 proof', which means it contains 40.0% ethanol by volume.