To which set or sets does - 20 belong

Natural numbers-counting numbers: 1,2,3,4,5 ...

whole numbers - natural numbers and zero

integers - whole numbers and negative numbers

ratioal numbers, number that can be written in form a/b where b≠0

irrational numbers - rational numbers and numbers that can't be written in form a/b, example is pi

real numbers is all of the above

complex numbers is none of the above, like √-1

so

-20 is an integer, a ratioal number, a real number