A glass prism has an angle of 60° and a refractive index of 1.50…

Step 1. In air. $\sin\dfrac{A + \delta_m}{2} = n\sin\dfrac{A}{2}$ $= 1.50 \times \sin 30^\circ = 0.750$. So $\dfrac{A + \delta_m}{2} = 48.6^\circ$ and $\delta_m = 2 \times 48.6^\circ - 60^\circ$ $= 37.2^\circ$.

Step 2. The angle of incidence. At the least deviation $i = \dfrac{A + \delta_m}{2} = 48.6^\circ$.

Step 3. The speed of light. $v = \dfrac{c}{n}$ $= \dfrac{3.00 \times 10^{8}\ \text{m/s}}{1.50}$ $= 2.00 \times 10^{8}\ \text{m/s}$.

Step 4. In water. The relative index is $\dfrac{1.50}{1.33} = 1.128$. Then $\sin\dfrac{A + \delta_m}{2} = 1.128 \times 0.5$ $= 0.564$, so $\dfrac{A + \delta_m}{2} = 34.3^\circ$ and $\delta_m = 2 \times 34.3^\circ - 60^\circ = 8.7^\circ$.