50х+10у+100-х-у=500
49х+9у-400=0
у=(400-49х)/9
то 50х+((400-49х)/9)-400=0
приводим к общему знаменателю
(450х+400-49х-3600)/9=0
401х-3200=0
401х=3200
х=3200/401=7(393/401)
у=(400-49*(7(393/401))/9=(400-(49*3200/401))/9=(400-(156800/401))/9= (3600/401)/9=400/401
проверка
50*(7(393/401))+10*(400/401)+100-(7(393/401)-(400/401)=500
(160000/401)+(4000/401)+(40100/401)-(3200/401)-(400/401)=500
200500/401=500
500=500 чтд
Ответ: х=7(393/401), у=400/401