A man standing in a point C of a ground. He observes a flying aero-plane in…
2020
A man standing in a point C of a ground. He observes a flying aero-plane in the east at an angle of elevation of 30° and after 30 seconds, he observes the same aero-plane in the west at an angle of elevation of 60°. If the aero-plane flies all along in a straight line at a height of 480√3 m, then its speed in km/h is:
- A.
247.8
- B.
257.5
- C.
230.4
- D.
249.8
Show answer & explanation
Correct answer: C
The height is 480√3 m. At 30°, the horizontal distance is 480√3 / tan 30° = 1440 m. At 60°, the distance is 480√3 / tan 60° = 480 m. Total distance in 30 seconds = 1920 m, so speed = 64 m/s = 230.4 km/h.