1000克饮料含甲为1000x/(x+y)克,含乙为1000y/(x+y)克
原来1000克饮料成本为：
1000x/(x+y)*5/500+1000y/(x+y)*4/500=10x/(x+y)+8y/(x+y)=(10x+8y)/(x+y)
现甲成本上升10%,乙下降10%后,甲每500克5.5元,乙每500克3.6元
现在1000克饮料成本为：
1000x/(x+y)*5.5/500+1000y/(x+y)*3.6/500=11x/(x+y)+7.2y/(x+y)=(11x+7.2y)/(x+y)
新饮料成本保持不变
则(10x+8y)/(x+y)=(11x+7.2y)/(x+y)
则10x+8y=11x+7.2y
则0.8y=x
所以x：y=0.8:1=4:5