The Rational Number Set Q

The Greeks soon discovered more mathematical concepts, like multiplication and division. Multiplication made sense.

What they also found is that they could express some numbers as a ratio of an integer and a natural number:One such example is -1.5, expressed as the integer -1, over the natural number 2:
This is a rational number.

It immediately follows that the Set of Integer Numbers Z is a subset of The Rational Number Set Q, therefore the Natural Numbers are also a part of the rational numbers:

Things get a bit more weird, with the introduction of:

The Set of Irrational Numbers I