$7\mathrm{.}14$ An investor bought $100$ shares of stock at a cost of $$10$ per share. He held the

stock for $15$ years and then sold it for a total of $$4,000.$ For the first three years,

he received no dividends. For each of the next seven years, he received total

dividends of $$50$ per year. For the remaining period, he received total dividends

of $$100$ per year. What rate of return did he make on the investment?