The refractive index of diamond is 2.42, and that of water is 1.33…

Step 1. (a) $v = \dfrac{3.00 \times 10^{8}\ \text{m/s}}{1.33} = 2.26 \times 10^{8}\ \text{m/s}$.

Step 2. (b) $n_{21} = \dfrac{2.42}{1.33} = 1.82$.

Step 3. (c) $\sin r = \dfrac{\sin 30^\circ}{1.82} = \dfrac{0.5}{1.82} = 0.275$, so $r = 15.9^\circ$.