- a. The monthly payment is approximately \( M \approx 496.42 \).
- b. The total paid since January 1 is approximately \( 5957.05 \).
- c. The percentage of the total paid that is interest is approximately \( 7.7\% \).
Thus, the final answers are:
\[
\boxed{M \approx 496.42}
\]
\[
\boxed{\text{Total Payment} \approx 5957.05}
\]
\[
\boxed{\text{Interest Percentage} \approx 7.7\%}
\]