Please help with these three parts

A $100 par value bond with 7% annual coupons and maturing at par in 4 years sells at a price to yield 6%. Determine the modified duration of the bond. (3.43) A company must pay liabilities of $1000 due one year from now and another $2000 due two years from now. There are two available investments: one-year zero coupon bonds and two-year bonds with 10% annual coupons maturing at par. The one year spot rate s 8% and the one-year forward rate is 9%, what is the company's total cost of the bonds required to exactly match the liabilities? (2625) A company must pay a benefit of S1000 to a customer in two years. To provide for this benefit, the company will buy one-year and three-year zero coupon bonds. The one-year spot rates are 8% and 10%, respectively. The company wants to immunize itself from small changes in interest rates on either side of 10% (Redington immunization). What amount should it invest in the one-year bonds? (421)