Mutual funds with assets in precious metals have displayed high price volatility in recent years. For example, the Sentry Precious Metals Growth Fund had annual returns in successive years from 2010 to 2015 inclusive of 78.5%, -23.1%, -7.9%, -51.4%, 3.0% and -6.8%, respectively. What was the fund's equivalent compound annual return for the six years ended December 31, 2015?