In the figure, let AB be the tower and C be the point on the ground from which the angle of elevation of the top of the tower AB is 60°. Join A and C and B and C. Then we get a right-angled triangle ABC with right angle at B and ∠ACB = 60°. Let D be the point 10 m vertically above C. Then, CD = 10 m. Given that the angle of elevation of the top of the tower from the point D is 30°. Join D and A. Also draw a line DE from D to AB, parallel to BC. Then we get a right-angled triangle AED with right angle at E and ∠ADE = 30°. We are to find the height of the tower, that is AB.

Clearly, ED = BC. Let AE = x

In ∆AED,

or,

or,

Again, in ∆ABC,

or,

or,

or,

3x = x + 10"

or,

x = 5m

Height of the tower = AB = AE + EB = 5m + 10m = 15m.