Please do it step by step

(CAS 97 Nov \#6) A loan is repaid by level payments of 1 at the end of each year for 10 years. At the time of the 4th regular payment, the borrower makes an additional payment equal to the amount of principal that, according to the original payment schedule, would have been repaid in the 5th regular payment. Payments of 1 continue to be made in the 5th and succeeding years until the loan is fully repaid. Determine the interest saved over the term of the loan. (A) 1v5 (B) 1v6 (C) 1v7 (D) i(v6+v7) (E) None of the above. [Ans: (B)]