Final answer:
Approximately 25% of banking assets in the United States are controlled by the 12 largest banks.
Step-by-step explanation:
Joseph Quinlan, the chief marketing strategist at Bank of America, estimated that approximately 25% of the banking assets in the United States are controlled by the 12 largest banks. According to the Dallas Federal Reserve, as of 2013, these 12 banks held 69% of all banking assets. This demonstrates the concentration and dominance of these large banks in the banking industry.