The American economy added 138,000 jobs in May and the unemployment rate dropped slightly to 4.3 percent, according to new numbers released Friday by the U.S. Bureau of Labor Statistics. Job gains occurred primarily in health care and mining, according to the bureau. The jobs gain was lower than the average monthly gain of 181,000 over the previous 12 months. Average hourly earnings for all non-farm employees rose by 4 cents, to $26.22, in May. Over the year, average hourly earnings have risen by 63 cents, or 2.5 percent.