Tính:

* Hướng dẫn giải

$a)\frac{2}{7}+\frac{4}{3}=\frac{6}{21}+\frac{28}{21}=\frac{34}{21}$

$b)\frac{23}{15}-\frac{3}{5}=\frac{23}{15}-\frac{9}{15}=\frac{14}{15}$

$c)\frac{9}{4}\times \frac{5}{7}=\frac{45}{28}$

$d)\frac{4}{9}:\frac{2}{5}=\frac{4}{9}\times \frac{5}{2}=\frac{20}{18}=\frac{10}{9}$