-- I borrow $2000, and 4 years later, I pay back the whole $2000.
-- I also pay another $1600 for keeping the money for 4 years.
-- $1600 is (1600/2000) = 80% of the $2000 that I borrowed and paid back.
-- To keep the money for 4 years, I paid an extra 80% of it.
-- I paid an extra (80/4) = 20% of the money for each year that I kept it.
-- That's a simple interest rate of 20% .
