Interest rates on short-term Treasury securities rose after falling sharply last week. The Treasury Department sold $6.4 billion in three-month bills at an average discount rate of 5.90%, up from 5.73% last week. Another $6.4 billion was sold in six-month bills at an average discount rate of 6.35%, up from 6.32% last week. The rates were the highest since Dec. 21, when three-month bills sold for 5.96% and six-month bills averaged 6.48%. The new discount rates understate the actual return to investors--6.09% for three-month bills, with a $10,000 bill selling for $9,850.90, and 6.67% for six-month bills selling for $9,679.00. In a separate report, the Federal Reserve said the average yield for one-year Treasury bills, the most popular index for making changes in adjustable-rate home mortgages, dropped to 7.15% last week from 7.19%.