An aircraft flying horizontally at height of 1.5 km above the ground is observed from certain point on the earth to subtend an angle of 60 degree. After 15 seconds, its angle of elevation from the same point is observed to be 30 degree. Calculate the speed of the aircraft in km/h.

Using tangent of the subtended angles you can determine the aircraft's horizontal distance for the two different angles.



The difference between those horizontal distances divided by the time of 15 seconds gives the speed that the aircraft is approaching.