How to reduce (simplify) ordinary fraction 21/4? Fraction can't be reduced (simplified). 21/4 = 5 1/4 = 5.25; Result written as a positive improper fraction, a mixed number (also called a mixed fraction) and a decimal number. Ordinary (simple, common) math fraction reducing to lowest terms (simplifying), answer with explanations belowHow to reduce (simplify) ordinary fraction: 21/4?