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Home Aptitude Height and Distance See What Others Are Saying!
  • Question
  • A man is observing from the top of a tower a boat speeding away from the tower. The boat makes an angle of depression of 45° with the man's eye when at a distance of 60 m from the tower. After 5 second, the angle of depression becomes 30°, Find the speed of the boat, assuming that it is running in still water.


  • Options
  • A. 30 km/hr.
  • B. 31.5 km/hr
  • C. 33 km/hr
  • D. 34 km/hr

  • Correct Answer
  • 31.5 km/hr 

    Explanation

    Let us draw the figure from the given question.
    Let AB = h meter be the height of the tower; C and D are the two points on the ground such that BC = 60 m; ?ACB = 45° and ?ADB = 30°
    Now from right triangle ABC,
    tan 45° = h/60
    ? 1 = h/60
    ? h = 60 m;
    Again from right triangle ABD;
    tan 30° = h/(x + 60)
    ? 1/?3 = 60/(x + 60)
    ? x + 60 = 60?3
    ? x = 60(1.73 - 1) = 43.8 meter
    Hence, speed of boat = 43.8/5 m/s = 43.8/5 x 18/5 = 31.5 km/hr.


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