21v2=u2-2as...........a is retardation
initial velo u
final velo wen the bullet passes through first plank is 19u20
s be length of one plank
2as=u2 -(19u20)2 ................i
let n be the planks after which through passing the velo of the bullet becomes 0
total distance=ns
final velo 0
so.... 2a(ns)=u2.......ii
dividing eq i and ii we get n=400/39 ≈11
