Rational numbers where the denominator is a multiple of the prime factors of the base have an exact fractional representation in that base. 1/3 doesn't have an

Rational numbers where the denominator is a multiple of the prime factors of the base have an exact fractional representation in that base. 1/3 doesn't have an exact representation in base 10 or base 2. 1/5th does have an exact representation in base 10 (0.2), but doesn't in base 2. 1/4th has an exact representation in base 10 (0.25) and in base 2 (0.01)