10%(x)+70%(350-x)=35%(350)
.1(x)+.7(350-x)=.35(350)
(.1x)+(245-.7x)=(122.5)
.1x+245-.7x=122.5
.1x-.7x=-122.5
-.6x=-122.5
.6x=122.5
x=204.1667 OR 204 and 1/6
350-x=145.8333 OR 145 and 5/6
Therefore in order to prepare 350ml of a 35% alcohol one would need 204.17 OR 204 and 1/6 ml of the 10% alcohol and 145.83 OR 145 and 5/6 ml of the 70% alcohol.