Convex lens of 10 cm focal length is combined with a concave lens of 6 cm focal length. Find the focal length of the combination.

 – 15 cm

The correct answer is option 2:  – 15 cm

$ f_1 = 10cm , f_2 = -6cm , F= ?$

$ \frac{1}{F} = \frac{1}{f_1} + \frac{1}{f_2} =\frac{1}{10} - \frac{1}{6} = \frac{3 - 5}{30} = \frac{-1}{15}$

$\Rightarrow F = -15cm$