Complex sentence: According to the official census of 1923 conducted by German authorities only 3.7 percent of city population was Polish (13,656 out of 366,730 citizens of the Free City) and in the 1920s and 1930s the city's population was over 90% German.
Split into simpler sentences: According to the official census of 1923 3.7 percent of city population was Polish (13,656 out of 366,730 citizens of the Free City). In the 1920s and 1930s the city's population was over 90% German.