Write down all the subsets of the following set: $\{a\}$

Show that the set of letters needed to spell "$CATARACT$" and the set of letters needed to spell "$TRACT$" are equal.

List all the elements of the following set:
$E = \{ x : x \text{ is a month of a year not having } 31 \text{ days} \}$

Find the pairs of equal sets,if any,give reasons:
$A = \{ 0 \}$
$B = \{ x : x > 15 \text{ and } x < 5 \}$
$C = \{ x : x - 5 = 0 \}$
$D = \{ x : x^2 = 25 \}$
$E = \{ x : x \text{ is an integral positive root of the equation } x^2 - 2x - 15 = 0 \}$

Let $A = \{1, 2, 3, 4, 5, 6\}$. Insert the appropriate symbol $\in$ or $\notin$ in the blank space:
$10 \dots A$

Which of the following are sets? Justify your answer.
The collection of all natural numbers less than $100$.