Homeowners seeking to refinance are keeping lenders busy as mortgage rates tumbled this week, reaching their lowest level since mid-2013. The benchmark 30-year fixed-rate mortgage fell to 3.85 percent from 3.99 percent last week, according to a national survey of large lenders. One year ago, that rate was 4.64 percent. Four weeks ago, it was 4.03 percent. The mortgages in this week's survey had an average total of 0.31 discount and origination points. Over the past 52 weeks, the 30-year fixed has averaged 4.29 percent. This week's rate is 0.44 percentage points lower than that 52-week average. The last time the 30-year fixed was lower was the week of May 22, 2013, when it was 3.74 percent.