Airport

Two french people are flying from France to Denmark by plane. Their friends who accompanied them stay at the airport.

1-a- In which referential must we be situated to have a man in the plane who is in mouvment ?

b- In which referential must we be situated to have a man in the plane who is immobile ?

2-a- In which referential must we be situated to have a man in the airport who is in mouvment ?

b- In which referential must we be situated to have a man in the airport who is immobile ?

The plane is heading towards Denmark at 900 km/h. A man in the airport is running at 12 km/h in the opposite direction.

3- What is the speed of the plane by report of the man ?