A boat has to go upstream from point "A" to point "B". The distance between point "A" and point "B" is 'D' kilometers. The velocity of stream and boat(w.r.t. ground) is same and equal to 'V' km/hour. The time taken by boat to reach point "B" is :

When a boat goes upstream, its effective speed is boat speed minus stream speed. If both are equal to V, the effective speed upstream becomes V - V = 0. This means the boat makes no progress relative to the ground and will never reach point B. The answer is physically correct - upstream travel is impossible when boat and stream speeds are equal.