Simplify the following text for a Grade 10 native English-speaking person.
Original: As a result of the building of two more developing areas ( Hellersdorf and Kaulsdorf ) the number of inhabitants increased so that the borough Hellersdorf was founded out of the three districts Hellersdorf , Kaulsdorf and Mahlsdorf on 1 June 1986 .
Simplified: The borough contains of five former villages : Biesdorf , Hellersdorf , Kaulsdorf , Mahlsdorf and Marzahn , which became part of Berlin in 1920 .