What set of numbers does 30 belong to?

Answer 1

To any set that contains it!

It belongs to {30},

or {45, sqrt(2), 30, pi, -3/7},

or all whole numbers between 23 and 53,

or multiples of 5,

or square roots of 900,

or composite numbers,

or counting numbers,

or integers,

or rational numbers,

or real numbers,

or complex numbers,

etc.