$\left \{ {{20x+15y=1600} \atop {4x=5y}} \right.$ ⇔$\left \{ {{20x+15y=1600} \atop {4x-5y=0}} \right.$ ⇔$\left \{ {{20x+15y=1600} \atop {12x-15y=0}} \right.$ ⇔$\left \{ {{20x+15y=1600} \atop {32x=1600}} \right.$ ⇔$\left \{ {{y=40} \atop {x=50}} \right.$
Chúc bạn học tốt nhé! ^^