An aeroplane flying horizontally at a height of 1500√3 m is seen at an elevation of 60° from a point on the ground. Fifteen seconds later its elevation from the same point is 30°. Its speed, in km/h, is:

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Accepted Answer

720 km/h. The horizontal distances are 1500√3/tan60° = 1500 m and 1500√3/tan30° = 4500 m, so the plane covers 3000 m in 15 s. That is 200 m/s, and 200 × 18/5 = 720 km/h. (200 is the speed in m/s, not km/h.)

An aeroplane flying horizontally at a height of 1500√3 m is seen at an elevation of 60° from a point on the ground. Fifteen seconds later its elevation from the same point is 30°. Its speed, in km/h, is:

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