A Bullet when fired at a target has its velocity decreased to 50% after penetrating 30 cm into it.   Additional thickness  it will penetrate in cm before coming to rest is,

apply work energy theorem, work done = change in KE Let  ‘F’ be resistive force, case 1:  F(30)  =   12 m (V24-V2) ---(1) case 2:   F(30+x)  =   12 m (0-V2) ---(2)  divide equation (1) by equation(2)  ⇒3030+x=34  ⇒x=10cm   _____1)