If a virtual image occurs at 20cm "behind" a concave mirror, would u put a screen 20cm before the mirror to obtain the clearest possible image? if not, where?

Answer 1

I wouldn't put up a screen at all. Real images can form images on screens, but virtual images cannot ever form an image on a screen. Virtual means it is created by the brain, in the brain, from difuse rays striking the eye. Think about that.