Question

A magnified, inverted image is located at a distance of 40.0 cm from a bi-convex lens with a focal length of 12.0 cm. The object distance is
17.2 cm
15.9 cm
10.5 cm
9.23 cm
7.05 cm

Answer 1

The object distance is 17.2 cm

As we know, the lens formula is $\frac{1}{f}$ = $\frac{1}{v}$ + $\frac{1}{u}$ 

where v = image distance from the lens
u = object distance from the lens
f = focal length of the lens 
Alos, focal length of convex lens is positive.

⇒ $\frac{1}{12}$ = $\frac{1}{40}$ +$\frac{1}{u}$

⇒ $\frac{1}{u}$ = $\frac{1}{12}$ - $\frac{1}{40}$

⇒ $\frac{1}{u}$ = $\frac{10-3}{120}$

⇒$\frac{1}{u}$ = $\frac{7}{120}$

⇒ u = $\frac{120}{7}$ = 17.2 cm