An object is placed at a distance of 30cm in front of a concave lens with a focal length of 15 cm. At what distance will the image form?

Focal length, f = -15 cm (Focal length of a concave lens is always negative.)
Object distance, u = - 30 cm (Object distance is always taken as negative.)
Image distance (v) = ?

Lens formula is given by:

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

Where u is the object distance, v is the image distance, and f is the focal length.

Putting the values in the  equation we get,

$\frac{1}{\text{v}}=-\frac{1}{15}-\frac{1}{30}$

$\frac{1}{\text{v}}=\frac{-2-1}{30}$= $\frac{-1}{10}$

$\therefore$v= -10 cm

Therefore, the image distance will be -10 cm which means the image will be formed on the left side of the mirror.