Answer:
Step-by-step explanation:
Let the distance be d . When sound travels d and is reflected back , it is heard by person as echo . Time taken by sound to travel total of distance 2 d 
= 2 d / v where v is velocity of sound. 
When this time is .05 s at least , echo will be heard . velocity of sound in air 340 m /s 
so for echo to be heard ,
2 d / 340 = .05 
d = 340 x .05 / 2 
= 8.5 m .