WASHINGTON (AP) — The average long-term U.S. mortgage rate fell this week, tracking Treasury yields that have retreated since a deal to end the war with Iran was announced. The benchmark 30-year fixed rate mortgage rate fell to 6.47% from 6.52% last week, mortgage buyer Freddie Mac said Thursday. One year ago, the average rate was 6.81%. Borrowing costs on 15-year fixed-rate mortgages, often sought by borrowers refinancing a home loan, also came down this week. That average rate fell to 5.81%