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What is the simplest form of $\frac{15}{20}$? 
Overseas Competition->Knowledge Point->Calculation Modules->Fractions->Basic Understanding of Fractions->Using Common Factors to Simplify Fractions
Factors of numerator $15$ are $1, 3, 5, 15$ and factors of denominator $20$ are $1, 2, 4, 5, 10, 20$.  The highest common factor(HCF) is $5$.  Divide $15$ and $20$ by the HCF:  $\frac{15 \div 5}{20 \div 5}$=$\frac{3}{4}$. 