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A person standing on an open ground hears the sound of a jet aeroplane, coming from north at an angle 60° with ground level. But he finds the aeroplane right vertically above his position. If v is the speed of sound, speed of the plane is :

1. v 2 3
2. 3 v 2
3. v
4. 2 v

Correct answer: 2 v

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