I just solved this problem. I was fooled by the statement:
Boeing 747 or an Airbus A380 (abbreviated 747 and 380 in air_routes)
I understood that to mean airplane would be either ‘747’ or ‘380’. This is false.
It would be helpful to state in the problem text that airplane might contain a list of types, e.g. ‘747 320 773’.
Thanks for a great course.