# How do you write the fraction 3/21 in simplest form?

$\frac{3}{21} = \frac{1}{7}$
$\frac{21}{3} = \frac{7}{1}$ -------> You know that $21 : 3 = 7$
You want $\frac{3}{21}$, so you must reverse it:
$\frac{3}{21} = \frac{1}{7}$