Complex sentence: The convention doubled attendance from the previous year, with 42,000 attending, nearly 40 percent coming from outside California, most were girls, and less than 10 percent were Korean.
Split into simpler sentences: The convention doubled attendance from the previous year, with 42,000 attending. Of those 42,000 nearly 40 percent come from outside California,, most were girls, and less than 10 percent were Korean.