A man is standing not exactly at the exactly at the centre, in between two parallel walls separated by a distance 990 m. He fires a gun and hears the first echo after 2 s, the time taken to hear 3rd echo after firing is ______ s. (take velocity of sound in air is `330 ms^(-1)`)

Options

A. 2
B. 6
C. 7
D. 8

Answer

B

Explanation

He hears the first echo after 2 s. The distance between the 1st wall and the man is
`d= v xx t= 330 xx 1 = 330 m`
So, the distance between the man and 2nd wall is
990 - 330 = 660 m.
So, time taken for 2nd echo time
`= ("distance ")/(" velocity ") = (2 x 660 )/(330 ) 4 s `
The 3rd and 4th echo reach at the same time. Time taken for the 3rd echo = time taken to hear the first echo + time taken to hear the 3rd echo
`= 2 + ( 2 xx 660 )/(330) = 2+2 xx 2= 6s `