The speed of a boat is $5km/h$ in still water it crosses a river of width $1km$ along the shortest possible path in $15$minutes. The velocity of the river water is

The width is 1 km and crossing along the shortest path takes 15 minutes, which is 0.25 hours, so the resultant velocity across the river is 1 divided by 0.25, or 4 km/h. Using Pythagoras theorem, the river velocity squared equals the boat velocity squared minus the resultant velocity squared. This gives 5 squared minus 4 squared, which is 9, so the river velocity is the square root of 9, or 3 km/h.

• Thanks
• Wrong explanation