A person standing on the bank of a river observes that the angle of elevation of the top of a tree on the opposite bank of the river is 60° and when he retires 40 meter away from the tree the angle of elevation becomes 30°. The breadth of the river is
- A. 60 m
- B. 30 m
- C. 40 m
- D. 20 m
Answer: D. 20 m
Explanation: Let river breadth be d and tree height be h. tan 60° = h/d = √3 ⇒ h = d√3. At distance d+40: tan 30° = h/(d+40) = 1/√3 ⇒ h√3 = d+40. Substituting h: 3d = d + 40 ⇒ 2d = 40 ⇒ d = 20 m.