tìm y: a) y:5/7=4/9; b) 7/9-y=5/9

* Hướng dẫn giải

$$\begin{gathered} a) y÷\frac{5}{7}=\frac{4}{9} \\ y=\frac{4}{9}+\frac{5}{7} \\ y=\frac{20}{36} \\ b) \frac{7}{9}-y=\frac{5}{9} \\ y=\frac{7}{9}-\frac{5}{9} \\ y= \frac{2}{9} \end{gathered}$$