A coin that was featured in a famous novel sold at auction in 2014 for $6,581,000. The coin had a face value of $10 when it was issued in 1791 and had previously been sold for $235,000 in 1968.
What annual rate did the 1968 buyer earn on his purchase? (Do not round intermediate calculations. Enter your answer as a percent rounded to 2 decimal places, e.g., 32.16.)