Let’s check each statement individually- Statement I- The speed of boat in downstream is 12 km/hr Here as we don’t know the speed of stream of water we cannot determine the speed of boat in still water. ∴ Statement I alone is not sufficient to find required answer. Statement II- The speed of boat in upstream is 4 km/hr Here as we don’t know the speed of stream of water we cannot determine the speed of boat in still water. ∴ Statement II alone is not sufficient to find required answer. By combining information of 2 statements- Let the speed of stream of water be x km/hr. & let the speed of boat in still water be y km/hr. The speed of boat in downstream is 12 km/hr. ⇒ Resultant speed will be addition of speed of boat in still water and speed of stream of water. ⇒ y + x = 12 km/hr ---- (1) & The speed of boat in upstream is 4 km/hr. ⇒ Resultant speed will be subtraction of speed of boat in still water and speed of stream of water. ⇒ y - x = 4 km/hr ---- (2) By adding result (1) & (2)- ⇒ (y + x) + (y – x) = 12 + 4 ⇒ 2y = 16 ⇒ y = 8 km/hr. ∴ Both statements are needed to find required answer.