What universal set $(s)$ would you propose for each of the following :

The set of isosceles triangles

Make correct statements by filling in the symbols $\subset$ or $ \not\subset $ in the blank spaces:

$\{a, b, c\} \ldots\{b, c, d\}$

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

$ 2 \, ……. \, A $

In each of the following, determine whether the statement is true or false. If it is true, prove it. If it is false, give an example.

If $A \subset B$ and $x \notin B,$ then $x \notin A$

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 $A$ ?