Financial math question: For question 2&3, did not understand how to calculate the book value. For example why is the periods calculated in question 2 for the book value at the end of 6 years is 4. Same question for question3. Can some explains the solution to me. Thank you!

2. Compute the book value at the end of 6 years (B6) and multiply by 6%: B6, 800aa 10,000v 10,693.02 (06) (10,693.02) 641.58 ANS. (B) 3. The book value just after the 4th coupon payment (i.e., the PV of future payments) at the orig- inal yield rate 60aa 1,000v6 at 4% l,104.84. The price to maintain the same yield rate (1,104.84)(1.04) 1,119.38. (2 months after the end of the 2nd year is -tof a semiannual interest period.) ANS. (E)