Hi, I'm preparing PART I for December 16th, Here is a question found in PRACTICE EXAM 2010 #25

Nicholas is responsible for the asset and liability management of JerseyBeech Bank, a small retail bank with USD 300 million in interest-bearing assets that yield approximately 70bp above LIBOR. The duration of the interest-bearing assets is 2.5 years. Due to the recent financial turmoil, the bank seeks to reduce potential negative impacts on earnings from adverse moves in interest rates. Thus, the bank decides to hedge 50% of its interest rate exposures using Treasury bond futures. Nicholas decides to use September T-bond futures that trade at 106-22 and will mature in three months; the cheapest-to-deliver bond associated with this contract is a 7-year, 10% coupon, with a current duration of 5 years. At the maturity of the futures contract, the duration of the bank's interest rate sensitive assets will not change; however the duration of the cheapest-to-deliver bond will fall to 4.9.
How many contracts should Nicholas buy or sell?

The answer is: Sell 717 contracts

我知道基本原理是被hedge的assets与duration乘积等于hedge的assets与duration乘积
但这道题里我找不出关键信息 不知道用哪个duration算

谢谢解答～