Study the graph and answer the following questions. The graph shows the number of bicycles parked in the parking space of a hall at various points of time.
What is the average number of parked cycles as seen from the graph?
The angle of elevation of an aeroplane as observed from a point 30 m above the transparent water-surface of a lake is 30° and the angle of depression of the image of the aeroplane in the water of the lake is 60°. The height of the aeroplane from the water-surface of the lake is