Answer:
19.5 m
Step-by-step explanation:
t = Time taken
u = Initial velocity = 0 (Assumed thrown from rest)
s = Displacement = 81 m
g = Acceleration due to gravity = 9.81 m/s² = a
Equation of linear motion

The time taken for the stone to reach the river is 4.06371 seconds
Distance = Speed×Time

The horizontal distance between the raft and the bridge when the child releases the stone should be 19.5 m