4%x=10%(x-y) ①
4%(x+300)=6.4%(x-y+300) ②
①式整理得:3x-5y=0 ③
②式整理得:8y-3x=900 ④
③+④得:3y=900
y=300
将y=300代入③:3x-1500=0
x=500