To find the focal length of a convex mirror, a student records the following data :The focal length of the convex lens is $f_1$ and that of mirror is $f_2$. Then taking index correction to be negligibly small, $f_1$ and $f_2$ are close to :

Question

$\begin{array}{1 1}(1) f_1=12.7\;cm \qquad f_2=7.8 \;cm \\ (2) f_1=7.8\;cm \qquad f_2=12.7 \;cm \\ (3) f_1=7.8\;cm \qquad f_2=25.4\;cm \\ (4) f_1=15.6\;cm \qquad f_2=25.4\; cm \end{array}$