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 $A$ ?

Which of the following are examples of the null set

Set of odd natural numbers divisible by $2$

Write down all the subsets of the following sets

$\{ a,b\} $

Which of the following are examples of the null set

$\{ x:x$ is a natural numbers, $x\, < \,5$ and $x\, > \,7\} $

The number of proper subsets of the set $\{1, 2, 3\}$ is

Write the following intervals in set-builder form :

$\left[ {6,12} \right]$