State which of the following sets are finite or infinite:
$\{ x:x \in N \text{ and } x^2 = 4 \}$

Show that $A \cup B = A \cap B$ implies $A = B$.

Find the union of each of the following pairs of sets:
$A = \{ x : x \text{ is a natural number and a multiple of } 3 \}$
$B = \{ x : x \text{ is a natural number less than } 6 \}$

For two sets $A$ and $B$,$A \cup B = A$ if and only if

Consider the sets $\phi, A=\{1,3\}, B=\{1,5,9\}, C=\{1,3,5,7,9\}$. Insert the symbol $\subset$ or $\not\subset$ between the pair of sets: $A, \ldots B$.

The number of non-empty subsets of the set $\{1, 2, 3, 4\}$ is