Let the speed of the boat in still water be b km/hr.
Speed of the current in the stream is 2 km/hr.
Therefore, speed of the boat in the upstream journey=(b-2)km/hr and downstream journey is (b+2)km/hr.
Time required in the upstream journey=12/(b-2) hrs
and time required in the downstream journey=12/(b+2) hrs
By the condition of the problem,
`rArr b=4, -1`
Since b is speed of the boat, it has to be non-negative.
So, b=4 km/hr.
Speed of the boat in still waters is 4 km/hr.