Let the acceleration provided by one plank be 'a'
The thickness of plank will be 'l'
Now, v2−u2=2as
If n no. of planks are required to stop the bullet:0−v2=2al
n=−v202al...(1)
velocity of bullet after passing though one plank,v′=v0−v020=19v020
After passing through one plank, v′2−v20=2as
361400v20−v20=2al
−39400v20=2al...(2)
putting (2) in (1): n=−v20−39v20×400
n=40039
Minimum 11 planks are needed.