Question

At what distance should an object be placed from a convex lens of focal
Length 18 cm to obtain an image at 24 cm from it on the other side. What
Will be the magnification produced in this case?

Answer 1

Use lenses formula
1/f = 1/v-1/u
1/18=1/24-1/u
U = - 72cm
Magnifications m = v/u =24/(-72)
-. 3
And image formed is inverted -ve sign show that

Answer 2

Focal length $\mathrm{f}=18 \mathrm{cm}$

Distance of image $v=24 \mathrm{cm}$

Using lens formula to find the position of object u.

According to lens formula,

$\frac{1}{f}=\frac{1}{v}-\frac{1}{u}$

Substituting the values  

$\frac{1}{18}=\frac{1}{24}-\frac{1}{u}$

u = -72 cm

Therefore the object must be placed at a distance of 72 cm from the focal length of the lens.

And the magnification of the image formed will be inverted this could be seen by the negative sign obtained. And this sign indicates the position in respect to the coordinate where the focal length of the lens is placed.