Функция х+у=400 0,12х+0,28у=64

Решите задачу:

$\left \{ {{x+y=400} \atop {0,12x+0,28y=64}} \right. \; \left \{ {{x+y=400} \atop {12x+28y=6400}} \right. \; \left \{ {{x+y=400} \atop {3x+7y=1600}} \right. \; \left \{ {{-3x-3y=-1200} \atop {3x+7y=1600}} \right. \; \; +\\\\ \left \{ {{x+y=400} \atop {4y=400}} \right. \; \left \{ {{x+y=400} \atop {y=100}} \right. \; \left \{ {{x=300} \atop {y=100}} \right.$