Make correct statements by filling in the symbols $\subset$ or $\not\subset$ in the blank spaces:
${ x:x \text{ is an equilateral triangle in a plane} } \dots { x:x \text{ is a triangle in the same plane} }$

Let $A = \{1, 2, \{3, 4\}, 5\}$. Which of the following statements are incorrect and why?
$1 \in A$

List all the elements of the following set:
$D = \{ x : x \text{ is a letter in the word } \text{"LOYAL"} \}$

Find the union of each of the following pairs of sets:
$A = \{ x : x \text{ is a natural number and a multiple of } 3 \}$
$B = \{ x : x \text{ is a natural number less than } 6 \}$

In this question,all integers are represented in base $10$. Consider the set $E$ of positive integers $n$ having the property that when any nonzero digit $d \in \{1, 2, \dots, 9\}$ is written to the right of $n$,the resulting number is divisible by $d$. Let $N$ be the smallest element of $E$. The product of the digits of $N$ is:

What universal set $(U)$ would you propose for each of the following:
The set of isosceles triangles