Final answer:
As of 2015, the Affordable Care Act greatly reduced the uninsured rate in the United States, with about 20 million Americans gaining coverage. This is evidenced by the decrease from 20.3% uninsured in 2012 to 11.5% in 2016.
Step-by-step explanation:
The question asks about the coverage percentage of Americans under the health exchanges implemented by the Affordable Care Act (ACA) in 2015.
According to the data, there was a significant decrease in the number of uninsured Americans from 20.3% in 2012 to 11.5% by the end of 2016, illustrating the impact of the ACA.
During this timeframe, it is estimated that 20 million Americans gained coverage through the ACA provisions. This illustrates the ACA's effectiveness in reducing the number of uninsured individuals and expanding health insurance coverage across the nation.