WASHINGTON (AP) — Interest rates on short-term Treasury bills were mixed in Monday's auction, with rates on six-month bills holding steady while rates on three-month bills dipped to their lowest level in five weeks. Separately, the Federal Reserve said Monday that the average yield for one-year Treasury bills, a popular index for making changes in adjustable rate mortgages, edged down to 0.80 percent on Friday, slightly below the 0.83 percent average seen on Feb.