A bullet emerges out from a stationary wooden plank with 75% of its initial kinetic energy. The number of additional planks required to stop the bullet is

Assuming constant retardation by the planks, 

From work energy theorem, 

$∆KE=W=-F_R\times t$

To loose 25% of the kinetic energy through 1 plank

$\Rightarrow -0.25KE=-F_{R}t$

Total number of planks required to loose the entire kinetic energy be n, then

$KE=n{F}_{R}t$

From the above equations, n = 4

Number of extra planks required is 3.