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Rational numbers are those that may be expressed as fractions. (Rational from the word "ratio" which is synonymous to fraction) Examples are 1/2 , 0.6 , 4, 0.1, 22/7 , etc.

Set-builder notation: 
S = { x | x ∈ R }

It would be impossible to list down all the elements of this set since this is an infinite set.