Complex sentence: Boeing designed the 747's hump-like upper deck to serve as a first--class lounge or extra seating, and to allow the aircraft to be easily converted to a cargo carrier by removing seats and installing a front cargo door.
Split into simpler sentences: Boeing designed the 747's hump-like upper deck to serve as a first-class lounge or extra seating. It also allowed the aircraft to be converted easily to a cargo carrier by removing seats and installing a front cargo door.