An investor holds the following portfolio of bonds: . $125,000 long in a 10-year zero-coupon bond; $75,000 long in a 15-year zero-coupon bond; $50,000 short in a 4-year zero-coupon bond; Assume that this investor always wants to keep her investments in the 10-year and 4-year bonds constant, but she is willing to change the amounts invested in the 15-year bond. How much should she invest or borrow in the 15-year zero coupon so that she holds a portfolio that has no exposure to interest rate risk