$\text{Đáp án+Giải thích các bước giải:}$
$\dfrac{a}{12}$ = $\dfrac{b}{9}$ = $\dfrac{c}{5}$ $=k$
Nên $a=12k,b=9k,c=5k ⇒ abc=12k.9k.5k=540k³=20$
$⇒ k³=$ $\dfrac{1}{27}$ $⇒k=$ $\dfrac{1}{3}$
$⇒ a=12.$ $\dfrac{1}{3}$ $=4$
$b=9.$ $\dfrac{1}{3}$ $=3$
$c=5.$ $\dfrac{1}{3}$=$\dfrac{5}{3}$