Asolution is 40% ethanol (density = 0.789 g/ml) by volume and 60% water (density = 1.00 g/ml). if ray lewis has 750.0 ml of said liquid, what percentage of the mass is ethanol?

Since the volume of ethanol is 40% of total volume, so V = 40% x 750 =300 ml 
volume of water = 750 - 300 = 450 ml

density of ethanol = m/vol = m/300 = 0.789 , m = 236.7 g
mass of water 450 g 
percentage of ethanol of total mass is 236.7/(450 + 236.7) x 100 = 34.5% approx