If $aN = \{ ax : x \in N \}$,then the set $3N \cap 7N$ is equal to $kN$,where $k$ is:

Write the following set in roster form:
$D = \{ x : x \text{ is a prime number which is a divisor of } 60 \}$

Which of the following is a set? Justify your answer.
$A$ collection of novels written by the writer Munshi Prem Chand.

Let $A = \{1, 2, 3, 4, 5, 6\}$. Insert the appropriate symbol $\in$ or $\notin$ in the blank space:
$8 \dots A$

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

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