Solving for both velocities:
16 km / 2 hours = 8 km per hour downstream
16 km / 4 hours = 4 km per hour upsteram
bsw means boat speed in still water cs means current speed
bsw - cs = 4 km per hour
bsw + cs = 8 km per hour
Adding both equaions:
2 bsw = 12 km per hour
boat speed = 6 km per hour
bsw + cs = 8 then
current speed = 2 km per hour
Source: 1728.com/veloccal.htm