This is the basic and exam oriented question as it was already asked in previous year paper of 2009 in which we have to find

(i) The horizontal distance between AB and CD,

(ii) the height of the lamp post,

(iii) the difference between the heights of the building and the lamp post

When from the top of a building AB, 60 m high, the angles of depression of the top and bottom of a vertical lamp post CD are observed to the 30° and 60° respectively.

Book RS Aggarwal, class 10, chapter 14, question no 33.

Hope it helps you..