9/m = 5/mn

$$\frac{9}{m} = \frac{5}{mn}$$ $$\frac{9}{m} = \frac{5}{mn}$$ $$\frac{9}{m} \cdot mn = \frac{5}{mn} \cdot mn$$ $$9n=5$$ $$\frac{9n}{9} = \frac{5}{9}$$ $$n = \frac{5}{9}$$ Ответ: $$n = \frac{5}{9}$$