Correct Answer - C
Let the thickness of eachof plank b s, If the initial speed of bullet is `100 ms^(-1)` then it stops by coverting a distance 2s. By applying `v^(2) = u^(2) - 2as`
`rArr 0 = u^(2)-2as`
` s = (u^(2))/(2a)` or `s alpha u^(2)` (if retardation is constant)
![image](https://learnqa.s3.ap-south-1.amazonaws.com/images/161127179077992557ClThSjNe0k7iIwiC.png)
if the speed of the bullet is double, then bullet will cover four times distance before coming to rest
i.e. `s_(2) = 4(s_(1) = 4(2s) rArr s_(2) = 8s`
So, number of planks requried = 8`