The pilot of an aircraft flies due north relative to the ground in a wind blowing 40 km/h toward the east. If his speed relative to the ground is 80 km/h, what is the speed of his airplane relative to the air?
1) 89 km/h
2) 85 km/h
3) 81 km/h
4) 76 km/h
5) 72 km/h
Answer: 1