(Bond valuation) Three years ago XYZ International issued some 28-year zero-coupon bonds that were priced with a market's required yield to maturity of 7 percent and a par value of $1,000. What did these bonds sell for when they were issued? Now that 3 years have passed and the market's required yield to maturity on these bonds has climbed to 9 percent, what are they selling for? If the market's required yield to maturity had fallen to 5 percent, what would they have been selling for?