Đáp án:
$\begin{array}{l}
\frac{{12a - 15b}}{7} = \frac{{20c - 12a}}{9} = \frac{{15b - 20c}}{{11}}\\
= \frac{{12a - 15b + 20c - 12a + 15b - 20c}}{{7 + 9 + 11}}\\
= \frac{0}{{27}} = 0\\
\Rightarrow \left\{ \begin{array}{l}
12a - 15b = 0\\
20c - 12a = 0\\
15b - 20c = 0
\end{array} \right. \Rightarrow \left\{ \begin{array}{l}
a = \frac{5}{4}b\\
c = \frac{3}{4}b
\end{array} \right.\\
a + b + c = 48\\
\Rightarrow \frac{5}{4}b + b + \frac{3}{4}b = 48\\
\Rightarrow 3b = 48\\
\Rightarrow b = 16\\
\Rightarrow \left\{ \begin{array}{l}
a = \frac{5}{4}b = 20\\
c = \frac{3}{4}b = 12
\end{array} \right.
\end{array}$
Vậy a=20; b=16; c=12
