A converging beam of rays is incident on a concave lens. After passing through the lens, the rays converge at a distance of 15 cm from the lens on the other side. If the lens is removed, the converging point of rays decreases by 5 cm. The focal length of lens is:

-30 cm

In the absence of concave lens the rays are focussed at 10cm .

$\Rightarrow u = +10 cm , V = 15 cm$

$\Rightarrow \frac{1}{f} = \frac{1}{v} - \frac{1}{u} = \frac{1}{15} - \frac{1}{10} = \frac{2-3}{30} = \frac{-1}{30}$

$ \Rightarrow f = -30cm$