How to solve: divide the number of specimen over the total area they take over. Then, divide both denominator and numerator by the total area.
1. Ferns: 30 ferns/15 m² --> divide by 15 to both --> 2 ferns/m²
2. Grass plants: 150 grass/15 m² --> divide by 15 to both --> 10 grass/m²
3. Oak trees: 6 tree/15 m² --> divide by 15 to both --> 0.4 tree/m²