One of the most important and exam oriented question as it is already asked in previous year paper of 2012 in which we are to calculate the height of the tower and the distance from a point A if the angle of depression from the top of a tower of a point A on the ground is 30 degrees.On moving distance of 20m from the point A towards the foot of the tower to a point B ,the angle of elevation of the top of the tower from the point B is 60 degrees.
Book RS Aggarwal, Class 10, chapter 14, question no 20
Hope it helps you.