Let velocity of light in air be v.
Velocity of light in medium, ${\mathrm{v}}_{\mathrm{m}}=\frac{\mathrm{v}}{\mathrm{n}}$ is the refractive index of the medium.

$\therefore 0.75\mathrm{v}=\frac{\mathrm{v}}{\mathrm{n}}\Rightarrow \mathrm{n}=\frac{4}{3}$ 

Given : n₁ = 1

Using Snell's law of refraction: $1\times \mathrm{sini}=\mathrm{n}\times \mathrm{sinr}$ ...................... (1)
But it is given that angle of incidence is very small, i.e $\mathrm{sini}\approx \mathrm{i}$
Hence angle of refraction is also very small $\Rightarrow \mathrm{sinr}\approx \mathrm{r}$ 

∴  From equation $\text{ (1), }\mathrm{i}=\frac{4}{3}\mathrm{r}$ 

Now angle of deviation,  $\mathrm{\delta }=\mathrm{i}-\mathrm{r}=\mathrm{i}-\frac{3}{4}\mathrm{i}\Rightarrow \mathrm{\delta }=\frac{\mathrm{i}}{4}$