From A to B
Speed of plane + Speed of air = Resultent speed
distance covering by plane 3000km in 4 hours
Resultent speed =3000/4=750 km/hr
So, Speed of plane = 750 - 50 =700km/hr
From B to A
Speed of plane - Speed of air = Resultent speed
Resultent Speed 3000/7=428.57km/hr
Speed of plane=428+50=478km/hr
So, Ans is (E) Cannot be determined