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.