Interest rates on short-term Treasury securities were mixed in Monday's auction, with rates on three-month bills declining while rates on six-month bills climbed to their highest level since mid-October. The Treasury Department sold $6.4 billion in three-month bills at an average discount rate of 5.96%, down from 6% last week. Another $6.4 billion was sold in six-month bills at an average discount rate of 6.48%, up from 6.45% last week. The rates on six-month bills were the highest since Oct. 19, when they averaged 7.21%. The three-month rate was the lowest since Dec. 7, when the rate averaged 5.81%.