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

Make correct statements by filling in the symbols $\subset$ or $\not\subset$ in the blank spaces:
${ x:x \text{ is an equilateral triangle in a plane} } \dots { x:x \text{ is a triangle in the same plane} }$

Let $A = \{1, 2, \{3, 4\}, 5\}$. Which of the following statements is incorrect and why?
${1, 2, 5} \in A$

If a set $A$ has $n$ elements, then the total number of subsets of $A$ is

In this question,all integers are represented in base $10$. Consider the set $E$ of positive integers $n$ having the property that when any nonzero digit $d \in \{1, 2, \dots, 9\}$ is written to the right of $n$,the resulting number is divisible by $d$. Let $N$ be the smallest element of $E$. The product of the digits of $N$ is:

Let $A = \{1, 2, \{3, 4\}, 5\}$. Is the statement $\{3, 4\} \in A$ correct or incorrect? Explain.