A nucleus has the mass number 64. Treat its mass as 64 u…

Step 1. The radius. $R = 1.2\ \text{fm} \times 64^{1/3} = 1.2\ \text{fm} \times 4 = 4.8\ \text{fm} = 4.8 \times 10^{-15}$ m.

Step 2. The volume. $V = \dfrac{4}{3}\pi R^3 = \dfrac{4}{3}\pi (4.8 \times 10^{-15}\ \text{m})^3 = 4.63 \times 10^{-43}\ \text{m}^3$.

Step 3. The density. $\rho = \dfrac{1.063 \times 10^{-25}\ \text{kg}}{4.63 \times 10^{-43}\ \text{m}^3} = 2.29 \times 10^{17}\ \text{kg/m}^3$.

Step 4. The cube. The volume is $(10^{-3}\ \text{m})^3 = 10^{-9}\ \text{m}^3$. The mass is $2.29 \times 10^{17}\ \text{kg/m}^3 \times 10^{-9}\ \text{m}^3 = 2.29 \times 10^{8}$ kg. That is $2.29 \times 10^{5}$ tonnes in a cube the size of a grain of salt.

Step 5. Compared with water. $\dfrac{2.29 \times 10^{17}\ \text{kg/m}^3}{1000\ \text{kg/m}^3} = 2.29 \times 10^{14}$.