Simplify the following text for a Grade 6 native English-speaking person.
Original: The total market capitalization of all publicly traded companies in the world was US$ 51.2 trillion in January 2007 and rose as high as US$ 57.5 trillion in May 2008 before dropping below US$ 50 trillion in August 2008 and slightly above US$ 40 trillion in September 2008 .
Simplified: The total market capitalization of all publicly traded companies in the world was US$ 51.2 trillion in January 2007 . In May 2008 to rose to US$ 57.5 trillion .