Complex sentence: Puerto Ricans have been immigrating to New York since 1838, in 1910 only 500 Puerto Ricans lived in New York, but by 1970 that number skyrocketed to over 800,000, and 40% of those lived in the Bronx.
Split into simpler sentences: Puerto Ricans have been immigrating to New York since 1838, though they did not arrive in large numbers until the 20th century. In 1910 only 500 Puerto Ricans lived in New York, but by 1970 that number had skyrocketed to over 800,000, and 40% of those lived in the Bronx.