You purchased a home on January 1, 1985 with a $100,000 mortgage requiring 30 equal annual payments at 10% interest with the first payment due on January 1, 1986. The bank sold your mortgage to an investor on January 1, 1992, immediately after receiving your 7th payment. The yield to the investor is 8%. Determine the banks overall return on its investment.

The answer is 11.6125%, but I'm not sure how to get there.