A 20-year callable bond has a maturity value equal to its par value of 1,000 and semiannual coupons paid at a coupon rate of 8% convertible semiannually. The bond may be called at the end of 12 years for a call price of 1,300. The bond may be called at the end of 15 years for a call price of 1,200. Finally, the bond may be called at the end of 18 years for a call price of 1,100. If the bond is to yield a return of 6% convertible semiannually, determine the following: (a) the optimal call date and the best price for the bond issuer. (b) the optimal call date and the best price for the bond purchaser