The focal length of a concave lens is 30 cm. At what distance should the object be placed from the lens so that it forms an image at 20cm from the lens?

Explanation

`1/v-1/u=1/f" or, "1/u=1/v-1/f`
`1/u-1/(-20)=1/(-30)=-1/20+1/30`
`1/u-(-3+2)/60`
`1/u-1/(-60)" or "u=-60` cm