Question: what is 20 lb paper?
Answer: The way we talk about paper in the United States is amazingly convoluted. The short answer is that 500 sheets of bond paper with a size of 17" by 22" have a weight of 20 pounds. The manufacturer would cut a sheet that big into four letter-size sheets, so a 500-sheet ream of 20-pound bond paper weighs 5 pounds.