Sara Blakely: 'How I invented Spanx'
9 March 2012 Last updated at 13:10 GMT
Forbes say there are now 1,210 dollar billionaires in the world.
One of those is Sara Blakely who invented the slimming knickers Spanx.
She is the world's youngest self-made billlionaire and she told BBC Radio 5 live how she came up with the idea for Spanx.
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