The average rate on a 30-year mortgage in the U.S. edged lower this week, ending a six-week climb. The rate slipped to 6.78% from 6.79% last week, mortgage buyer Freddie Mac said Thursday. That’s still down from a year ago, when the rate averaged 7.4%. Borrowing costs on 15-year fixed-rate mortgages, popular with homeowners seeking to refinance their home loan to a lower rate, also eased this week.