Question

a stone is dropped freely in the river from a bridge. it takes 5sec to touch the water surface in the river . calculate; [a] the height of the bridge from water level . [b] the distance covered by stone in the last second [g=9.8m/s2

Answer 1

Refer to the file attached below.
Hope this helps.

Answer 2

Given : 
Initial speed=u= 0 m/s
time=t=5sec
Case 1)
distance travelled = height of bridge=h m
From second equation of motion:
s=ut+1/2at²h
=0xt +1/2 9.8 x 5 x5h
=122.5m
∴The height of the bridge from water level  is 122.5m

 Case ii)
Distance travelled in 4sec
= (1/2x9.8x4x4)
=16x4.9
=78.4 m
Distance travelled in last second :
 =Distance travelled in 5sec - distance travelled in 4 sec=122.5 -78.4=44.1m
∴The distance covered by stone in the last second is 44.1m