$A = \{ x:x \ne x\} $ represents

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

$1 \subset A$

Are the following pair of sets equal ? Give reasons.

$A = \{ x:x$ is a letter in the word ${\rm{FOLLOW }}\} $

$B = \{ y:y$ is a letter in the word $WOLF\} $

Let $A=\{1,2,3,4,5,6\} .$ Insert the appropriate symbol $\in$ or $\notin$ in the blank spaces:

$ 2 \, ....... \, A $

Let $S=\{1,2,3, \ldots, 40)$ and let $A$ be a subset of $S$ such that no two elements in $A$ have their sum divisible by 5 . What is the maximum number of elements possible in $A$ ?

In rule method the null set is represented by