A bullet is moving with velocity u after emerging from a plank

Question

its velocity reduces by u/20 then find the total number of planks required to stop this bullet?

Answer 1

u - u/20 = 19u/20 = .95u

.95² = .9025

the bullet loses about 10% of its energy (kinetic), as work passing through the plank

it should stop in the 11th plank