A person standing on the bank of a river observes that the angle subtended by a tree on the opposite bank is 60°. When he retires 40 m from the bank, he finds the angle to be 30°. The breadth of the river is :

• ক 30 m
• খ 20 m ✅ সঠিক উত্তর
• গ 40 m
• ঘ 60 m