1) First you need to find the monthly payment.
[tex]m = P\frac{i(1+i)^{360}}{(1+i)^{360} - 1}, P = 120,000, i = 0.005 \\ \\ m = 719.46[/tex]
Next, you need a function for balance remaining at time 't'.
[tex]B(t) = (P-\frac{m}{i})(1+i)^t +\frac{m}{i} \\ \\ B(180) = 85,258.8[/tex]
So after 15 years, she still owes 85,258.80 on the mortgage.