U.S. mortgage rates fell more than three-quarters of a percentage point Tuesday after the Federal Reserve said it will buy as much as $600 billion of debt.

The average U.S. rate for a 30-year fixed mortgage ended the day at about 5.5 percent after falling to as low as 5.25 percent, according to Bankrate Inc. It was 6.38 percent Tuesday morning, North Palm Beach, Florida-based Bankrate said, based on a wider sampling than the so-called overnight rate published on its Web site.

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