Speed of the boat with current = (4+c) ft/s
Speed of the boat against current = (4-c) ft/s
Where, c= speed of the current
Speed = Distance/Time => Time = Distance/Speed
In both cases, time is equal
Therefore,
48/(4+c) = 16/(4-c) => 48(4-c)=16(4+c) => 192-48c = 64+16c => 192-64 = (48+16)c => 128 = 64 c => c= 128/64 = 2 ft/s
The current is travelling at 2ft/s