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:  $\frac{15 \div 5}{20 \div 5}$=$\frac{3}{4}$. 
What is the simplest form of $\frac{15}{20}$? 
Options:
A: $\frac{4}{5}$ 
B: $\frac{3}{4}$ 
C: $\frac{2}{3}$ 
D: $\frac{15}{20}$ 