7. There are three bonds in a portfolio. The term structure of interest rates is flat. Each bond has semiannual coupons and is redeemable at face value. The bonds are: A 10-year bond with face value of $1,000, a current market value of $1,100, and a Macaulay duration of 6.50. A 20-year bond with face value of $1,000, a current market value of $900, and a Macaulay duration of 13.00. A 30-year bond with face value of $1,000, a current market value of $1,200 and a Macaulay duration of 18.40. Find the Macaulay duration of this three-bond portfolio