I'd like to discuss this Venn Diagram

Question

I'm very confused about this question. I would like to check if i'm correct or not.

Question:

For each $n \in \mathbb{N}$ be $ D_n = (0, {1 \over n})$, where $(0, {1 \over n})$ represents the open interval which extremes are $0$ and ${1 \over n}$. Define the difference between $D_3 - D_{20}$

My solution:

I tried to put the sets in a Venn Diagram to make it clear but sincerely i got even more confused.

Thanks in advance.