A rifle bullet loses 1/20th of its velocity in passing through a plank. The least number of such planks required just to stop the bullet is

Let the thickness of one plank is s if bullet enters with velocity u then it leaves with velocity v=u−u20=1920ufrom v2=u2−2as⇒1920u2=u2−2as  ⇒40039=u22asNow if the n planks are arranged just to stop the bullet then again from v2=u2−2as0=u2−2ans⇒n=u22as=40039⇒n=10.25As the planks are more than 10 so we can consider  n = 11