Cách làm cho bạn:
$\frac{3}{5}:\frac{6}{7}=\frac{3}{5}\times \frac{7}{6}=\frac{21}{30}=\frac{7}{15}$
$\frac{2}{7}:\frac{8}{5}=\frac{2}{7}\times \frac{5}{8}=\frac{10}{56}=\frac{5}{28}$
$\frac{4}{15}:\frac{2}{5}=\frac{4}{15}\times \frac{5}{2}=\frac{20}{30}=\frac{2}{3}$
$\frac{7}{9}:\frac{21}{9}=\frac{7}{9}\times \frac{9}{21}=\frac{63}{189}=\frac{1}{3}$
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