Question

a point light source is kept

Answer 1

we know

1/f = 1/u + 1/v

where

u = object distance
v = image distance

here the object is kept at focus
we know when an object is kept at focus the image formed is virtual and object distance is less than image distance

1/40 = - 1/40 + 1/v

1/v = 1/40 + 1/40

1/v = 2/40

1/v = 1/20

v = 20