An object has a density of 0.9 kg/m3. Calculate the fraction of its volume submerged in freshwater (density 1.0 kg/m3) and in seawater (density 1.03 kg/m3).

Step 1: Apply the principle of flotation.
For a floating object, the weight of the object is equal to the weight of the fluid displaced. This means the buoyant force equals the object's weight.

$$F_B = W_{obj}$$ $$\rho_{fluid} V_{submerged} g = \rho_{obj} V_{obj} g$$

The fraction of the object's volume that is submerged is given by the ratio of the object's density to the fluid's density:

$$\frac{V_{submerged}}{V_{obj}} = \frac{\rho_{obj}}{\rho_{fluid}}$$

a)

Step 2: Calculate the fraction of the object's volume submerged in freshwater.
Given:
Density of object, $\rho_{obj} = 0.9 \, \text{kg/m}^3$
Density of freshwater, $\rho_{fw} = 1.0 \, \text{kg/m}^3$

$$\frac{V_{submerged}}{V_{obj}} = \frac{0.9 \, \text{kg/m}^3}{1.0 \, \text{kg/m}^3}$$ $$\frac{V_{submerged}}{V_{obj}} = 0.9$$

The fraction of the object's volume submerged in freshwater is $\boxed{\text{0.9}}$.

b)

Step 3: Calculate the fraction of the object's volume submerged in seawater.
Given:
Density of object, $\rho_{obj} = 0.9 \, \text{kg/m}^3$
Density of seawater, $\rho_{sw} = 1.03 \, \text{kg/m}^3$

$$\frac{V_{submerged}}{V_{obj}} = \frac{0.9 \, \text{kg/m}^3}{1.03 \, \text{kg/m}^3}$$ $$\frac{V_{submerged}}{V_{obj}} \approx 0.873786$$

Step 4: Convert the fraction to a percentage.
Percentage submerged $= \left( \frac{V_{submerged}}{V_{obj}} \right) \times 100%$$

$$\text{Percentage submerged} = 0.873786 \times 100\%$$ $$\text{Percentage submerged} \approx 87.38\%$$

The percentage volume of the object submerged in seawater is $\boxed{87.38\%}$.