What set of numbers does 45 belong to?

Answer 1

To any set that contains it!

It belongs to {45},

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

or all whole numbers between 43 and 53,

or multiples of 5,

or composite numbers,

or counting numbers,

or integers,

or rational numbers,

or real numbers,

or complex numbers,

etc.