Complex sentence: It was found that during this time 392 Mooney-related accidents had occurred, 75% of which were due to human error, and approximately six Mooney aircraft out of every 100 registered in the U.S. had been involved in an accident.
Split into simpler sentences: It was found that in this time 392 Mooney-related accidents had occurred, 75% of which were due to human error. Approximately six Mooney aircraft out of every 100 registered in the U.S. was involved in an accident.