The Treasury Department sold $5.5 billion in three-month bills an average discount rate of 5.60%, down from 5.70% last week. Another $5.5 billion was sold in six-month bills at an average discount rate of 5.60%, down from 5.73% 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 5.36% on July 25, 1977. The new discount rates understate the actual return to investors-- 5.76% for three-month bills and 5.84% for six-month bills.