Dear Ram Sai Charan,

Ans:- Let the initial velocity is U the final velocity is V Then from the given relation we have V²=0.75U²

or V=√3/2U

Let the retardation for each block is =f

then V²=U² - 2f(9x)

where x=thickness of each plate

So U²/4=18fx

Let the total no of block required to stop it from it's initial velocity is n

Then U²=2fnx

or   72fx=2fnx

or   n=36

Hence the excess no of plates required=36-9=27(ans) 

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