The number of proper subsets of the set $\{1, 2, 3\}$ is
Write the following intervals in set-builder form :
$\left( {6,12} \right]$
Write the solution set of the equation ${x^2} + x - 2 = 0$ in roster form.
Write down all the subsets of the following sets
$\{ 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 $A$ ?