$2a=3b \to \dfrac{a}{3}=\dfrac{b}{2}$
$5b=7c \to \dfrac{b}{7}=\dfrac{c}{5}$
$\to \dfrac{a}{21}=\dfrac{b}{14}=\dfrac{c}{10}$
$\to \dfrac{3a}{63}=\dfrac{7b}{98}=\dfrac{5c}{50}$
Áp dụng tính chất dãy tỉ số bằng nhau:
$\dfrac{3a}{63}=\dfrac{7b}{98}=\dfrac{5c}{50}=\dfrac{3a+5c-7b}{63+50-98}=\dfrac{30}{15}=2$
$\to \begin{cases}a=2.21=42\\b=2.14=28\\c=2.10=20\end{cases}$
Vậy $(a,b,c)=(42,28,20)$
