The answer is 2 raccoons per square meter.
The math is very clear. It can be solved using proportion.
If 20 raccoons live in 10 square meters, how many of them will live in 1 square meter:
20 raccoons : 10 m² = x raccoons : 1 m²
⇒

⇒ x =

⇒ x = 2
So, a population density is 2 raccoons per square meter.