An object is placed on the axis of a convex lens. Its real image is formed 80 cm from the object. The magnification is 3. The focal length of the lens is

The correct option is C 15 cm
According to the question,
v – u = 80 cm …..(i)
Magnification, $m=\frac{v}{u}=-3$$\Rightarrow v=-3u....\left(i\right)$
Using (i), and (ii),
–3u – u = 80
⇒ u = –20 cm
v = 60 cm [From (i)]
Using lens formula,
$\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$$\Rightarrow f=\frac{u\times v}{u-v}$$\Rightarrow f=\frac{(-20)\times 60}{(-20)-60}=15cm$
Hence, the correct answer is option (3).