Consider a two-step mortgage for $150,000, 30 years, monthly payments, an initial interest rate of 5%, a cap of 5%, and a single rate adjustment at the end of year 7. Assume that the index rate at the end of year 7 is 5% and the margin is 2%. If the borrower pays an extra $100 with each payment starting in month 85, by how many months will he shorten the term of the loan?