Which of the following are sets ? Justify your answer.
A collection of most dangerous animals of the world.
If $Q = \left\{ {x:x = {1 \over y},\,{\rm{where \,\,}}y \in N} \right\}$, then
Write the set $\{ x:x$ is a positive integer and ${x^2} < 40\} $ in the roster form.
Let $A, B,$ and $C$ be the sets such that $A \cup B=A \cup C$ and $A \cap B=A \cap C$. Show that $B = C$
The number of elements in the set $\{x \in R :(|x|-3)|x+4|=6\}$ is equal to