Giải hệ x+y=720 và $\frac{x}{10}$+$\frac{3y}{25}$=800

$\left \{ {{x+y=720} \atop {\frac{x}{10}+ \frac{3y}{25}}=800} \right.$

⇔$\left \{ {{5x+5y=3600} \atop {5x+6y=40000}} \right.$

⇔$\left \{ {{5x+5y=3600} \atop {5x+6y- (5x+5y)=40000- 3600}} \right.$

⇔ $\left \{ {{x+y=720} \atop {y=36400}} \right.$

⇔$\left \{ {{x=-35680} \atop {y=36400}} \right.$

Vậy $\left \{ {{x=-35680} \atop {y=36400}} \right.$