Simplify the following text for a Grade 9 native English-speaking person.
Original: After the transfer , the municipality of Rio de Janeiro became the Estado da Guanabara ( State of Guanabara ) , which existed from 1960 until 1975 when the State of Guanabara and the State of Rio de Janeiro merged and assumed the name of Rio de Janeiro State , having as capital the city of Rio de Janeiro .
Simplified: Afterwards , the municipality of Rio de Janeiro became the Estado da Guanabara ( State of Guanabara ) , from 1960 until 1975 . In 1975 , the State of Guanabara and the State of Rio de Janeiro merged , to be named the state of Rio de Janeiro .