Question

A man fired a bullet against a wall and hears an echo after 2 s. He walks 80 m towards the wall and fired bullet, such that he hears echo after 1 s. Find the distance from wall to the second fired place.

• A. 140 m
• B. 150 m
• C. 80 m
• D. 170 m

Answer 1

The correct option is C 80 m

Let the distance from wall to the first fired place be 'd' m.
Then the time after which echo heard is 2 s.
$\therefore V=\frac{2d}{t}=\frac{2d}{2}$
$V=d$ ........... (1)
He walked 80 m then the distance of person from wall to the second fired place is (d-80)m and time taken for echo to be heard at this point is 1 s.
$V=\frac{2(d-80)}{1}$
$V=2(d-80)$ .............. (2)
From (1) and (2)
$d=2d-16)$
$d=160m$
The distance from wall to the second fired place is (d-80) $=$ 160 - 80 $=$ 80 m.