The Treasury sold $7.4 billion in new three-month bills at an average discount rate of 5.78%, down from 5.85% last week. Another $7.4 billion was sold in new six-month bills at an average discount rate of 5.80%, down from 5.85% last week. The rates for three-month bills were the lowest since Sept. 2, 1977, when they averaged 5.55%. Rates for six-month bills were the lowest since they sold for 5.68% on Aug. 8, 1977. The new discount rates understate the actual return to investors--5.95% for three-month bills and 6.06% for six-month bills. The discount rate reflects the price discount received when the securities are purchased at less than face value.