liability problem

A liability consists of a series of 15 annual payments of 35,000 with the first payment to be made one year from now.



The assets available to support this liability are five-year and ten-year zero-coupon bonds.



The annual effective interest rate used to value the assets and liabilities is 6.2%. The liability has the same present value and duration as the asset portfolio.



Calculate the amount invested in the five-year zero-coupon bonds.



I have no idea how to approach this problem.



The solution first finds the PV of the liability, which is 335530.30



Next it finds the duration of the liability, which is 6.89214



Then it assumes X is the amount invested in the 5 year bond and finds X by



**(X(5) / 335530.30) + (1 - X/335530.30)*10 = 6.89214**

and X is 208556



I was wondering why you had to find the duration of the liability for this problem and what the starred equation represents



thanks

liability problem