Question

A point object is placed ar a distance of 25cm from a convex lens of focal length 20cm

Answer 1

your question is incomplete. A complete question is -----> A point object is placed at a distance of 25cm from a convex lens of focal length 20cm. Where will the image form at?

solution, focal length of lens , f = 20cm [ given lens is convex lens.]
object distance , u = -25cm

use formula,
1/v - 1/u = 1/f

1/v - 1/-25 = 1/20

1/v = 1/20 - 1/25 = 1/100

v = 100cm

hence, image is formed at 100cm, if object is placed at 25cm from a convex lens of focal length 25cm .