Let us assume that the motion of the bullet is of constant retardation inside the water.
Initial kinetic energy 21mv02=90J
⇒v0=30ms−1
Let the retardation is a, then after 1s
21m(30−a×1)2=40J
⇒30−a=20
⇒a=10ms−2
Stopping distance for the constant retardation is given by,
l=2au2=20302=45m.
Therefore, the minimum length of the pool required is 45m.
