In November 2009, Accel Growth Fund invested $20 million in Groupon by buying 2,932,553 shares of Series E convertible preferred stock. What was the realized IRR of this investment, if the fund sold at IPO and "each share of Series E preferred stock was converted into 12 shares of Class A common stock on October 31, 2011" (Amendment No. 8 to form S-1, filed with SEC on November 2, 2011, page II-3)?