Đáp án: $\left\{\begin{array}{l}x=420\\y=300\end{array}\right.$
Giải thích các bước giải:
$\left\{\begin{array}{l}x+y=720\\(x+\frac{3}{20}x)+(y+\frac{3}{25}y)=819\end{array}\right.\\\Leftrightarrow \left\{\begin{array}{l}x+y=720\\\frac{23}{20}x+\frac{28}{25}y=819\end{array}\right.\\\Leftrightarrow \left\{\begin{array}{l}x=420\\y=300\end{array}\right.$