Let $A=\{1,2,\{3,4\}, 5\} .$ Which of the following statements are incorrect and why ?
$\varnothing \in A$
Which of the following are sets ? Justify your answer.
The collection of all the months of a year beginning with the letter $\mathrm{J}.$
Let $A=\{1,2,3,4,5,6\} .$ Insert the appropriate symbol $\in$ or $\notin$ in the blank spaces:
$10 \, .........\, A $
Write the following as intervals :
$\{ x:x \in R, - 12\, < \,x\, < \, - 10\} $
Which of the following are sets ? Justify your answer.
The collection of all natural numbers less than $100 .$