$(4\mathrm{.}2\mathrm{.}4)$The amortization schedule for a $100,5\%$ bond with yielding a nominal annual rate of $i^{(2)}=6\mathrm{.}6\%,$ gives a book value of $90$ for the bond at the beginning of a certain $6-$month period just after a coupon has been paid. What is the book value at the start of the next $6-$month period?