A ferry boat has an internal volume of 1 m^3 and a weight of 50 kg...

By the principle of flotation, the weight of the boat is balanced by the buoyant force.

$mg = V_{in} \rho_w g$

where $m = 50 \text{ kg}$ is the mass of the boat, $V_{in}$ is the immersed volume, and $\rho_w = 1000 \text{ kg/m}^3$ is the density of water.

$50 = V_{in} \times 1000$

$V_{in} = \dfrac{50}{1000} = \dfrac{1}{20} \text{ m}^3$

The total volume of the boat is $V = 1 \text{ m}^3$.

The fraction of the boat's volume immersed in water is $\dfrac{V_{in}}{V} = \dfrac{1/20}{1} = \dfrac{1}{20}$.