what sets or set dose15 belong to

Answer 1

an infinite number of sets have 15 as an element

{15}, {1,15}, {24,23,15, 584} or

{1/4, 2.5, pi, e, sqr2, 3.6, 15}

{red, blue, house, horse, war, peace, senator, president, 15}

any set that has 15 as one of the elements.

the other elements can be any numbers, colors, objects, people or anything, and as many as you want.

but maybe there's more in the question than given

15 is an element of any set of odd numbers

an element of any set of numbers divisible by 15

among other endless sets with 15 in them somehow, some way