A bullet loses 1/20th of its velocity after passing through a plank . How many planks it can pass so

let the length of the plank be x

using v^2 = u^2 + 2aS find the value of v.

v = u-(1/20)u =19/20u

Find a

a = 39/800 (u^2/x)

Now for bullet to stop completely, v = 0

0 = u^2 -39/800 (u^2/x).S

s=800/39x which means approximately 20.5 planks