A $15-$year bond has annual coupons. The annual coupon starts at $$15,$ and decreases by $$1$ each year. The last coupon at the end of $15$ years is $$1$ and the redemption value at maturity is $$100.$ If the bond is bought to yield $7\%$ effective per annum, compute the portion of the ninth coupon used to amortize the book value.

$[$Total: $6$ marks$]$