Correct answer: (b) 30

Explanation:

Lens formula,

\(\frac{1}{v}\) - \(\frac{1}{u}\) = \(\frac{1}{f}\)

\(\frac{1}{v}\) - \(\frac{1}{-30}\) = \(\frac{1}{10}\)

\(\frac{1}{v}\) = \(\frac{1}{10}\) - \(\frac{1}{30}\)

⟹ \(\frac{1}{v}\) = \(\frac{3-1}{30}\)

v = 15 cm

Now we have u = +10 cm

\(\frac{1}{v}\) - \(\frac{1}{10}\) = \(\frac{1}{-10}\)

⟹ \(\frac{1}{v}\) = \(-\frac{1}{10}\) + \(\frac{1}{10}\)

⟹ v = ∞

It means for the third lens, rays are coming parallel to the principal axis, hence, an image will be formed at the focus of the third lens.

Thus, V = + 30 cm (from third lens)