Calculate the volume of liquid X.

The mass of the bottle when full of water is $80 \text{ g}$.
The mass of the empty bottle is $40 \text{ g}$.

$$\text{Mass of water} = \text{Mass of bottle full of water} - \text{Mass of empty bottle}$$ $$\text{Mass of water} = 80 \text{ g} - 40 \text{ g}$$ $$\text{Mass of water} = 40 \text{ g}$$

A) Mass of water: $\boxed{40 \text{ g}}$

The mass of the bottle when full of liquid X is $75 \text{ g}$.
The mass of the empty bottle is $40 \text{ g}$.

$$\text{Mass of liquid X} = \text{Mass of bottle full of liquid X} - \text{Mass of empty bottle}$$ $$\text{Mass of liquid X} = 75 \text{ g} - 40 \text{ g}$$ $$\text{Mass of liquid X} = 35 \text{ g}$$

B) Mass of liquid X: $\boxed{35 \text{ g}}$

The density of water is given as $1 \text{ g/cm}^3$.
The mass of water is $40 \text{ g}$ (from Step 1).

$$\text{Volume of water} = \frac{\text{Mass of water}}{\text{Density of water}}$$ $$\text{Volume of water} = \frac{40 \text{ g}}{1 \text{ g/cm}^3}$$ $$\text{Volume of water} = 40 \text{ cm}^3$$

C) Volume of water, given its density is $1 \text{ g/cm}^3$: $\boxed{40 \text{ cm}^3}$

The volume of the water that fills the bottle is equal to the volume of the bottle.

$$\text{Volume of the bottle} = \text{Volume of water}$$ $$\text{Volume of the bottle} = 40 \text{ cm}^3$$

D) Volume of the bottle: $\boxed{40 \text{ cm}^3}$

The mass of liquid X is $35 \text{ g}$ (from Step 2).
The volume of liquid X is equal to the volume of the bottle, which is $40 \text{ cm}^3$ (from Step 4).

$$\text{Density of liquid X} = \frac{\text{Mass of liquid X}}{\text{Volume of liquid X}}$$ $$\text{Density of liquid X} = \frac{35 \text{ g}}{40 \text{ cm}^3}$$ $$\text{Density of liquid X} = 0.875 \text{ g/cm}^3$$

E) Density of liquid X: $\boxed{0.875 \text{ g/cm}^3}$