Rifleman standing between two cliffs hears two echos from a single shot

[GforOevOerD]

Summary:: Hello.There is a rifleman standing between two cliffs with vertical walls, and the distance of closer cliff is 700 meters. If he shoots and run toward the closer cliff ( 700 meters ) and hears the first echo after 4 (s) and second echo after 2 (s) of the first echo, what is the distance of two cliffs?

Hello.There is a rifleman standing between two cliffs with vertical walls, and the distance of closer cliff is 700 meters. If he shoots and run toward the closer cliff ( 700 meters ) and hears the first echo after 4 (s) and second echo after 2 (s) of the first echo, what is the distance of two cliffs?

A : 1700

B : 1740

C : 1750

D : 1690

I can solve when the man is just standing, but not moving. However, I don't know what to do when he is running. I'd be so grateful if you could help me.

 

[berkeman]

Welcome to PF

Please show us your solution for when the Rifleman is stationary. Then we can discuss which variable needs to be modified when he is moving. Thanks.

 

[rsk]

In the first 4s, how far does he run? How much less has the sound traveled when he hears that first echo? (just let his running speed = v for now). How does that change what you did for the case where he is stationary?

After 6s, when he hears the 2nd echo, how far has he run? How does that adjust your working from the stationary case?(There may be a better way, I had to assume a speed for sound and it turns out he's running quite fast...)