A rifle bullet loses 1/20th of its velocit

Let the thickness of one plank is $s$ if bullet enters with velocity $u$ then it leaves with velocity 

$v = \left( {u - \frac{u}{{20}}} \right) = \frac{{19}}{{20}}u$ from ${v^2} = {u^2} - 2as$ 

$⇒$ ${\left( {\frac{{19}}{{20}}u} \right)^2} = {u^2} - 2as$

$⇒$ $\frac{{400}}{{39}} = \frac{{{u^2}}}{{2as}}$ 

Now if the n planks are arranged just to stop the bullet then again from ${v^2} = {u^2} - 2as$ $0 = {u^2} - 2ans$ 

$⇒$ $n = \frac{{{u^2}}}{{2as}} = \frac{{400}}{{39}}$

$⇒$ $n = 10.25$ As the planks are more than $10$ so we can consider $n = 11$