[tex]\left \{ {{x+y=120} \atop {y* \frac{20}{100} =x}} \right. <=> \left \{ {{x+y=120} \atop {y* \frac{1}{5} =x}} \right.<=> \left \{ {{x+y=120} \atop {\frac{y}{5} =x}} \right.[/tex] <=>
[tex]<=> \left \{ {{x+y=120} \atop {y=5x}} \right.<=> \left \{ {{x+5x=120} \atop {y=5x}} \right.[/tex]<=>
[tex]<=> \left \{ {{6x=120} \atop {y=5x}} \right.<=> \left \{ {{x=20} \atop {y=5*20}} \right.<=> \left \{ {{x=20} \atop {y=100}} \right.[/tex]
