To calculate the planting density (seeds per acre), you divide the total number of seeds by the total area in acres.
You have 8,000 seeds and 0.4 acres:
\[ \text{Density} = \frac{\text{Total Seeds}}{\text{Total Area (acres)}} = \frac{8,000 \text{ seeds}}{0.4 \text{ acres}} = 20,000 \text{ seeds per acre} \]
So the planting density is 20,000 seeds per acre.