Fringe width is directly proportional to wavelength and inversely proportional to refractive index.  

The formula is given by, 

In air, Fringe width, x = $\frac{D\lambda }{d}$ 

In medium, $\mathrm{Fringe} \mathrm{width}, x = \frac{\mathrm{D\lambda }}{\mathrm{\mu d}}$
where,
d is the distance between the slits and, 
D is the distance between the screen and the slit. 

Therefore, fringe width decreases by a factor of $\mathrm{\mu } (1.33)$ when placed in the liquid. 

In the case of convex lens, 

Object distance, u = -15 cm
Focal length, f = 10 cm 

Now, using the lens formula, 

                   $\frac{1}{\mathrm{f}} = \frac{1}{\mathrm{v}}-\frac{1}{\mathrm{u}}$

                   $\frac{1}{\mathrm{v}} = \frac{1}{\mathrm{f}}+\frac{1}{\mathrm{u}}$ 
                        $= \frac{1}{10}+\frac{1}{-15}$

                         = $\frac{3-2}{30} =\frac{1}{30}$ 

i.e., the image is formed at a distance of 30 cm. 

For convex mirror,

Object distance, $\mathrm{u} = 30 - 10 = 20 \mathrm{cm}$ 
Image distance, $\mathrm{v} = 10 + 15 = 25 \mathrm{cm}$ 

Now, using mirror formula,

               $\frac{1}{\mathrm{f}} = \frac{1}{\mathrm{v}}+\frac{1}{\mathrm{u}}$ 

                    $= \frac{1}{25}+\frac{1}{-20}$ 

                    $$\begin{gathered} = \frac{4-5}{100} \\ =\frac{-1}{100} \end{gathered}$$
i.e.,            $\mathrm{f} = -100 \mathrm{cm}.$ 

Therefore, focal length of the convex mirror is 100 cm.