Question

The speed of a stream is 4km/h. A boat travels 6 km upstream in the time it takes to travel 12 km downstream. What is the speed of the boat in still water

Answer 1

6 km × 1 hour/(b-4 km) = 12 km × (1 hour/(b+4 km) 

6/(b-4) hours = 12/(b+4) hours

6(b+4) = 12(b-4)

6b + 24 = 12b - 48
72 = 6b
b = 12 
The boat's speed in still water is 12 km/hour.