Cách làm cho bạn:
$\frac{3}{5}+\frac{5}{8}=\frac{24}{40}+\frac{25}{40}=\frac{49}{40}$
$\frac{21}{40}-\frac{3}{8}=\frac{21}{40} -\frac{15}{40}=\frac{6}{40}$
$\frac{5}{7}\times \frac{12}{11}=\frac{5\times 12}{7\times 11}=\frac{60}{77}$
$\frac{15}{7}:\frac{5}{14}=\frac{30}{14}:\frac{5}{14}=\frac{30:5}{14}=\frac{6}{14}=\frac{3}{7}$
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