A bullet fired into a wooden block loses 75% of its velocity after penetrating 15 cm into it' How much further distance it has to penetrate to be stopped completely ?

Let u be the initial velocity of the bullet and a be the retardation of the wooden board assumed to be constant. Using the formula v2 =u2 +2as, we haveu42=u2−2a×15oru2−16u216=−30a∴u2=32a...(1)Let it travel a further distance s before it is stopped. Then0=u42−2asor u2 = 32 as … (2)From eqs. (1) and (2).32 a s = 32 a or s = 1 cm