Please show the solution:

Twelve years ago, an investor bought a certain number of shares of stock for which he paid P 100 per share. At the end of each year, for the next 6 years, he received dividends of P 20 per share. Thereafter he received P 10 per share at the end of each year for the next 6 years. If he sells this stock now at P 90 per share, what rate of return will he receive?

Ans: 16.88%