A Mexican billionaire tops Forbes' world's richest list for a fourth year in a row.
6. David Koch (tie) -- The billionaire is co-owner (with older brother Charles) and an executive vice president of Koch Industries, a conglomerate that is the second-largest privately held company in the U.S. He's worth an estimated $34 billion.
Wisconsin Dept. of Natural Resources
Enrique Castro-Mendivil / Reuters
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Larry Downing / Reuters
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