First of all, let us calculate the annual gains: they are 80000-60000=20000$. In three years, the profit will be 3*20000=60000$. Hence, the break-even investment would be 60000$. For a year after that, the profit will be 20000$; hence the return on investment would be 20000/60000=33,33% per year. After 6 years, the investment would have yielded a 100% profit (return on investment).