Q: A bullet leaving the muzzle of a rifle barrel with a velocity v penetrates a plank and loses one-fifth of its velocity. It then strikes second plank, which it just penetrates through. The ratio of the thickness of the planks, supposing the average resistance to the penetration is same in both the cases is
(a) 5 : 9
(b) 9 : 16
(c) 9 : 5
(d) 16 : 9
Ans: (b)