Frequency density - 

is proportional to the frequency per unit for the data in each class.

Population density - 

is the number of people living per unit of area.

Substance density - 

is measured as the substance's mass per unit of volume.

$$0 < w \leq 5$$ - 

Frequency density = 4

$$5 < w \leq 15$$ - 

Frequency density = 2

$$15 < w \leq 30$$ - 

Frequency density = $$\frac{4}{3}$$

$$0 < l \leq 15$$ - 

Frequency density = 1.6

$$15 < l \leq 20$$ - 

Frequency density = 4

$$20 < l \leq 30$$ - 

Frequency density = 1.4

$$30 < l \leq 35$$ - 

Frequency density = 3.2