WASHINGTON — Interest rates on short-term Treasury bills were mixed in Monday’s auction with rates on three-month bills unchanged and rates on six-month bills at the highest level in four weeks.

The Treasury Department auctioned $32 billion in three-month bills at a discount rate of 0.075 percent, unchanged from last week. Another $28 billion in six-month bills was auctioned at a discount rate of 0.110 percent, up from 0.095 percent last week.