Which group of sets does the number -15 belong to?

Answer 1

To any set that contains it!

It belongs to {-15},

or {sqrt(2), -15, pi, -3/7},

or all whole numbers between -43 and 53,

or multiples of 5,

or composite numbers,

or integers,

or rational numbers,

or real numbers,

or complex numbers,

etc.

Answer 2

Whole numbers