Given data:
- Water content of saturated soil = 25%
- Bulk unit weight of soil = 20 kN/m^3

To find:
- Dry unit weight
- Void ratio
- Specific gravity of solids

Solution:

1. Dry unit weight:
The dry unit weight of soil can be calculated using the following formula:

γ_dry = γ_b / (1 + w)

where,
γ_dry = dry unit weight of soil
γ_b = bulk unit weight of soil
w = water content of soil

Substituting the given values, we get:

γ_dry = 20 / (1 + 0.25) = 16 kN/m^3

Therefore, the dry unit weight of soil is 16 kN/m^3.

2. Void ratio:
The void ratio of soil can be calculated using the following formula:

e = w / (1 - w)

Substituting the given values, we get:

e = 0.25 / (1 - 0.25) = 1/3

Therefore, the void ratio of soil is 1/3.

3. Specific gravity of solids:
The specific gravity of solids is defined as the ratio of the mass of solids to the mass of an equal volume of water at a specified temperature. It can be determined by conducting a specific gravity test on the soil sample.

Alternatively, if we know the dry density and void ratio of the soil, we can use the following formula to calculate the specific gravity of solids:

G_s = (γ_dry / e) / [(γ_w / e) + 1]

where,
G_s = specific gravity of solids
γ_dry = dry unit weight of soil
e = void ratio of soil
γ_w = unit weight of water (assumed to be 9.81 kN/m^3)

Substituting the given values, we get:

G_s = (16 / (1/3)) / [(9.81 / (1/3)) + 1] = 2.70

Therefore, the specific gravity of solids is 2.70.

Conclusion:
- Dry unit weight of soil = 16 kN/m^3
- Void ratio of soil = 1/3
- Specific gravity of solids = 2.70