What is the minimum distance required to hear an a echo In air?

What is the minimum wind velocity required to obtain useful energy with a windmill?

What is the minimum distance between an object and its real image in the case of a concave mirror ?

On a morning walk, three persons walk with steps measuring 45 cm, 42 cm, and 40 cm respectively. What is the minimum distance each should walk to cover the same distance in complete steps?

What is the minimum distance for clear visibility in human eyes?

What is the minimum number of vectors required to give zero resultants?