Density is defined as mass per unit volume. The formula for density is:

$$Density = \frac{Mass}{Volume}$$

Given:

Mass of the cork block (m) = 400 grams = 0.4 kg

Volume of the cork block (V) = 10 cm³ = 0.000010 m³

Substitute the values in the formula:

$$Density = \frac{0.4 kg}{0.000010 m³} = 40,000 kg/m³$$

Therefore, the density of the small cork block is 40,000 kg/m³.