Question

SUNIL is standing between two walls. The wall closest to him is at a distance of 360meters. If he shouts he hears the first echo after 4second and another after another 2 second What is the velocity of sound in air? What is the distance between the two walls

Answer 1

Distance between the closest wall and Sunil = 360m
Time after which he hears echo = 4s
Velocity of sound = 2(Distance/time)
                            = 360*2/4 
                            = 180m/s
Let distance between two walls be (x)m 
Distance of another wall from Sunil = (x - 360)m
Time after which he hears echo = 4s + 2s = 6s
Velocity of sound = 2(Distance)/time
                            = 2(x-360)/6
Hence,
180 = 2(x-360)/6 (Velocity of sound remains constant)
⇒1080 = 2(x-360)
⇒540 = x - 360
⇒x = 540 + 360
Distance between two walls=900m 

Answer 2

The sound first travels from Sunil to first wall and then travels back to him.

So total distance travelled by sound= 2*360=720

Time taken by Sound= 4 Seconds

So speed Of sound= Distance traveled /Time taken=720/4=180m/s

Distance traveled by second echo= 6*180 =1080 m

Distance of 2nd wall from Sunil= 1080/2=540 m

Distance between two walls= 540-360=180m