+) Có $\dfrac{x}{3}=\dfrac{y}{4}$ $⇒$ $\dfrac{x}{3}.\dfrac{1}{5}=\dfrac{y}{4}.\dfrac{1}{5}$$⇒$$\dfrac{x}{15}=\dfrac{y}{20}$
+) Có $\dfrac{y}{5}=\dfrac{z}{7}$$⇒$$\dfrac{y}{5}.\dfrac{1}{4}=\dfrac{z}{7}.\dfrac{1}{4}$$⇒$$\dfrac{y}{20}=\dfrac{z}{28}$
CHÚC BẠN HOK TỐT!!!