甲种酒精溶液中有酒精6升,水9升；乙种酒精溶液中有酒精9升,水3升.要配制成50％的酒精溶液7升,问两种酒精溶液各需多少

6/(6+9)=40%
9/(9+3)=75%
(75%-50%):(50%-40%)=5:2
甲种:7*[5/(5+2)]=5升
乙种:7*[2/(5+2)]=2升