A concave lens has a focal length of 15 cm. At what distance should the object be from the lens placed so that if forms an image 10 cm from the lens?

The correct option is A

30 cm

A concave lens always forms a virtual, erect image on the same side as the object.

Image distance, n = - 10 cm

Focal length, f = - 15 cm

Object distance, u = ?

Using the len's formula, $\frac{1}{v}$ - $\frac{1}{u}$ = $\frac{1}{f}$ we get

$\frac{1}{-10}$ - $\frac{1}{u}$ = $\frac{1}{-15}$

u = - 30 cm

Thus, the object should be placed 30 cm on the lens.