The intensity of a sound gets reduced by 20% on passing through two consecutive slabs is :

Options

A. 0.4
B. 0.36
C. 0.3
D. 0.6

Answer

B

Explanation

Let initial intensity = I.
after passing through one slab `I_(1) = I - (20)/(100) I = (4I)/(5)` .
after passing through `2^(nd)` slab
`I_(2) = I_(2) = (4I)/(5) - (20)/(100) ((4I)/(5)) = (16 I)/(25)` .
% decrease in intenity = `(I - I_(2))/(I) xx 100`
= `(I - ((16 I)/(25))/(I) ) xx 100 = 36%`.
Hence, the correct choice is (b).