If an object is placed 10 cm in front of a convex mirror of focal length 20 cm, then distance of the image from the mirror is:

The correct option is B $\frac{20}{3}cm$
Given; Focal length, $f=20cm$
Object distance, $u=-10cm$

Using mirror formula.
$\frac{1}{v}+\frac{1}{u}=\frac{1}{f}$
$\frac{1}{v}=\frac{1}{f}-\frac{1}{u}$
$=\frac{1}{20}-\frac{1}{-10}$ ($\because u=-10$)
$=\frac{1}{20}+\frac{1}{10}$
$=\frac{3}{20}$

$\Rightarrow v=\frac{20}{3}$cm