An object is placed at a distance of 1.5 m from a screen and a convex lens is interposed between them. The magnification produced is 4. What is the focal length of the lens.

Answer 1

M= -di/do

solve for di.

then
1/f=1/di+1/do solve for f

Answer 2

m = 4
= v/u =4
v = 4u
then,
1/f = 1/v - 1/u
substitute v with 4u,
1/f = 1/4u - 1/u
1/f = 1/4u - 4/4u
1/f = -3/4u
=f=-4u/3
total dist,
u + 4u = 1.5 * 100 (m-cm)
5u = 150
u = 150/5 = 30cm

substituting u in -4u/3

-4 * 30/3
-120/3 = -40cm

hope so you got it
this might be the ans

Answer 3

but the answer of this question in my guide is something else...not at all related to the one answered above