REALTALK

(х+2y)/y = 5; x/y + y = 5; x/y = 5-y; x=y(5-y); y = x /(5-y)

y/x = x/(5-y) / x = x^2/(5-y)     ; тройная дробь, х переходит в числитель

(3х + у) / y = 3y(5-y)/y = 2y(5-y) = 10y - 2y^2