An object is placed 40 cm in front of a convex mirror of radius of curvature 20 cm. The image:

is virtual and 8 cm behind the mirror

According to new Cartesian sign conventions,

u = -40 cm, R = +20 cm

f = $\frac{R}{2}$ = +10 cm

According to mirror formula

$\frac{1}{u}+\frac{1}{v}=\frac{1}{f}$   ∴  $\frac{1}{-40}+\frac{1}{v}=\frac{1}{10}$

$\frac{1}{v}=\frac{1}{10}+\frac{1}{40}=\frac{1}{8}$   or   v = 8 cm

The image is formed at 8 cm behind the mirror. It is a virtual image.