The U.S. unemployment rate fell to a seven-year low in August as employers added a modest 173,000 jobs, a key piece of evidence for the Federal Reserve in deciding whether to raise interest rates from record lows later this month. The Labor Department said Friday that the jobless rate fell to 5.1 percent — a level consistent with a normal economy and the lowest since April 2008 — from 5.3 percent in July. Though hiring in August was the slowest in five months, the government revised up job growth for June and July by a combined 44,000. From June through August, the economy added a robust 221,000 jobs a month, up from an average of 189,000 from March through May.