A boat goes 15 km upstream and 10 1/2 km down stream in 3 hrs 15 min. It goes 12 km upstream and 14 km downstream in 3 hrs. What is the speed of the boat in still water?
The angle of elevation of the top of a pillar from the foot and the top of a building 20 m high, are 60° and 30° respectively. The height of the pillar is