I'm not sure where my teacher got n=61 from can someone please explain it to me. Thank you!

AM5a How long to reduce principal by half Your friend is taking out a mortgage for $170,000 at 9.20% repayable with monthly payments over 15 years. She respects your financial expertise and asks "how many payments will I have to make before I reduce the principal balance by half its original amount." You pull out your calculator, and tell her the number of payments she'll make to reduce the balance by half is a. 108 b. 81 c. 89 d. 98 e. 119 2 2. 119 /E 34