Simplify the following text for a Grade 15 native English-speaking person.
Original: He worked full time for the Cowles Commission until October 1950 until January 1951 , he was a visiting professor , teaching at classes that were taught before by Professor Koopman in the Department of Economics , and he was a very important professor there .
Simplified: Joining full time in October 1950 until January 1951 , he was a visiting professor , assuming Koopman 's classes in the Department of Economics , and led the commission 's research on theory of resource allocation .