Cách làm cho bạn:
a.
b.
$\frac{9}{20} - \frac{8}{15}\times \frac{5}{12}=\frac{9}{20}-\frac{40}{180}=\frac{81}{180}-\frac{40}{180}=\frac{41}{180}$
$\frac{2}{3}:\frac{4}{5}:\frac{7}{12}=\frac{2}{3}\times \frac{5}{4}\times \frac{12}{7}=\frac{120}{84}=\frac{20}{14}$
$\frac{4}{9}+\frac{11}{8}-\frac{5}{6}= \frac{32}{72}+\frac{99}{72}- \frac{60}{72}=\frac{51}{72}=\frac{17}{24}$
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