Final answer:
A 2012 Kelton Research poll indicated that 36 percent of adult Americans, equating to about 80 million people, believed that aliens have visited Earth. There is no specific percentage provided for those who believe in alien experiments conducted by the U.S. government at Area 51.
Step-by-step explanation:
According to a 2012 Kelton Research poll, 36 percent of adult Americans believed that alien beings have actually landed on Earth. When considering the U.S. adult population in 2012, which was approximately 222,000,000 (2.22 × 108), we can calculate that about 7.99 × 107 adults believed that aliens have visited Earth. This is roughly 80 million people who held this belief.
However, there isn't a direct answer to how many believe that the U.S. government has performed alien experiments at Area 51. Such specific polls may exist, but they are not cited here. Conspiracy theories, urban legends, and stories such as the "Face on Mars," and rumors about Roswell and Area 51 persist and can influence public belief about extraterrestrial activity associated with the government.