Question

An bullet can travel in the air at a speed of 1000 kmph. When walls of 10 cm thickness are placed in its path, the speed at which the bullet emerges out of the last wall in its path diminishes by a quantity which varies as the square root of the number of walls placed in its path. With four walls its emergent speed is 300 kmph. The greatest number of walls that can be placed in front of the bullet so that the bullet emerges out of the final wall in its path is :___

Answer 1

Decrease in speed =1000-300 = 700 kmph.
700 = $k\sqrt{4}\Rightarrow$ k= 350
Now, for the speed to decrease to zero.
1000 = $k\sqrt{4}=350\sqrt{x}$, where x is the number of walls in the way which will cause the speed of the emergent bullet to become 0.

x=8.16
So,if 8 walls are placed in the bullet's way, there will be some residual emergent speed.If 9 walls are placed, it will not emerge out of the 9 th wall.