Question 318343
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.