A) 1500
B) 750
C) 720
D) 1440
Correct Answer: D
Solution :
We find the horizontal distance covered by the projection of the plane on the ground in the time given. We assume that the height at which the plane was flying above ground is constant at \[\sqrt{3}kms\]. In the first case, distance of projection of plane from point of observation is \[\frac{\sqrt{3}}{\tan 60{}^\circ }=1km\]. In the second case, the distance of the projection of the plane from the point of observation is \[\frac{\sqrt{3}}{\tan 30{}^\circ }=3km\]. So, a distance of \[3-1=2km\]. is covered in 5 seconds. So the speed of the plane is \[\frac{2\times 3600}{5}=1440km/hr\]. So option D is the correct answer.
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