Tim x

* Hướng dẫn giải

$\begin{array}{l}a)x\times \frac{2}{5}=\frac{6}{7}\\ x=\frac{6}{7}:\frac{2}{5}\\ x=\frac{6}{7}\times \frac{5}{2}\\ x=\frac{15}{7}\end{array}$