You bought a house 5 years ago. At the time you took out a mortgage worth $200,000. The mortgage was set up to pay off the loan through monthly payments over twenty years. The Bank charged a rate of 8.50% with monthly compounding. There is no penalty for prepayment. Your income has gone up in the last five years so that you are considering paying more. (The Bank will take the PV of the stream of expedited payments you offer). [Prepayment option]

Suppose you decide to start paying twice the original payment each month. In how many years will the remaining loan be fully paid off?