Complex sentence: By that time, less than 5 percent of the people from Chimbote would truly consider themselves native; between 1960 and 1970, Chimbote's population multiplied by more than a hundred times.
Split into simpler sentences: By that time, less than 5% of the people from Chimbote could consider themselves as truly native. The reason was that between 1960 and 1970, Chimbote multiplied its population more than a hundred times.