Answer:
Stone falls in air for t = 7 s
and stone falls in water for t = 5 s
Step-by-step explanation:
As we know that average speed of the stone in air is given as

average speed of stone in water is given as

total time of the motion of stone in water and air is given as

total distance from the cliff to the seabed is given as

let the stone moves in air for time "t"
and stone mover in water for total time "12 - t"
now we have




so stone falls in air for t = 7 s
and stone falls in water for t = 5 s