## Thinking Mathematically (6th Edition)

$\emptyset \in \{\emptyset, \{\emptyset\}\}$ is true.
$\emptyset \in \{\emptyset, \{\emptyset\}\}$ This is correct. Let's say: $B = \{\emptyset, \{\emptyset\}\}$ $\in$ means that the element to the left is contained in the set to the right. We can see that $\emptyset$ is contained in $B$. Therefore, we can say: $\emptyset \in B$