one beer , or one shot , is not 20g of pure alchihol
http://www.cdc.gov/alcohol/faqs.htm
What is a standard drink in the United States?
A standard drink is equal to 13.7 grams of pure alcohol or
12-ounces of beer.
8-ounces of malt liquor.
5-ounces of wine.
1.5-ounces or a “shot” of 80-proof distilled spirits or liquor (gin, rum, vodka, whiskey, etc).