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 = -60cm`