a, 7/15+4/5;b.5/7:8/3;c. 7/3*5/2-5/14

* Hướng dẫn giải

$a)\frac{7}{15}+\frac{4}{15}=\frac{11}{15}$

$b)\frac{5}{7}÷\frac{8}{3}=\frac{5}{7}\times \frac{8}{3}=\frac{15}{56}$

$c)\frac{3}{7}\times \frac{5}{2}-\frac{5}{14}=\frac{15}{14}-\frac{5}{14}=\frac{10}{14}=\frac{5}{7}$