Correct Answer - C
Let (u) be the initial velocity of bullet and (a) be the retardation of bullet while passign through a plank. Velocity of bullet after passing theough plank
` v= u - u/(20) = (19 u)/(20)`
Taking motion of bullet through one plank and using the relation, ` v^2 =u^2 +2 as`, we hve
(19)/(20) u)^@ = u^2 -2 aS` or ` 2 aS =u^2 - ((10)/(20)u)^2`
[ :. motion is retarted ]`
` 2 a S=u^2 - ( 361)/(400)u^2 = (39u^2)/(400)` ltbRgt Taking motion bullet through (n) plaks
` ` v=0, S=n S`
` 0 u^2 -2 a nS`
or ` n=u^2/(2 aS0 = u^2 /( 39u^2 //400) = (400)/(39) = 10.25 ~~ 11`.
