Rational number

A rational number is any number that can be expressed as the quotient or fraction (a/b) of two integers, with the denominator not equal to zero. Any rational number with a denominator whose only prime factors are 2 or 5 may be precisely expressed as a decimal fraction.

Example:
1/2=0.5
1/20=0.05
1/5=0.2
1/4=0.25
1/25=0.04

But if the rational numbers denominator has any prime factors other than 2 or 5, decimal fraction cannot be expressed as finite decimal expansion, and has a unique eventually repeating infinite decimal expansion.

Example:

1/3=0.333333… (with 3 repeating)
1/9=0.111111… (with 1 repeating)