Consider a $542,000, 7.42%, 30 year mortgage that was originated 30 years ago. Except for three curtailments at the end of months 142, 152, and 162, the borrower made all payments according to schedule until the mortgage was prepaid in full at the end of year 25. Oddly, the curtailment at the end of month 142 happened to equal (to the closest penny) the interest paid during year 1. Furthermore, the curtailment made at the end of month 152 happened to equal (to the closest penny) the principal paid during months 19-31. Moreover, the curtailment made at the end of month 162 happened to equal (to the closest penny) the interest paid during months 155 to 160.

What is the borrowers' effective borrowing rate?