The average interest rate on 30-year fixed-rate mortgages slipped to 7.88% for the week, down from 7.94% last week, Freddie Mac said. A year ago, the rate on 30-year mortgages stood at 7.82%. Fifteen-year mortgages, a popular refinancing option, dipped to an average 7.6% this week from 7.65% the week before. On one-year adjustable-rate mortgages, lenders were asking an average initial rate of 7.26%, down from last week's 7.33%. For the same week last year, one-year ARMs averaged 6.22%. The rates do not include add-on fees, known as points.