sorry can you explain what the correct solution is. Do you need to know how much the density of alcohol is rel. water? Also water and alcohol volumes don't add as far as I remember, so you need to account that that too?
Not sure what you mean by volumes "not adding". One way to calculate it is like:
density_alcohol = 0.789g/ml
density_water = 1g/ml
weight_total = 16g
(density = weight / volume)
volume_alcohol = 0.4 * volume_total
volume_water = 0.6 * volume_total
weight_total = weight_alcohol + weight_water
weight_alcohol =
density_alcohol * volume_alcohol =
0.789 * volume_alcohol =
0.789 * (0.4 * volume_total) =
0.3156 * volume_total
weight_water =
density_water * volume_water =
volume_water =
0.6 * volume_total
weight_total =
0.3156 * volume_total + 0.6 * volume_total =
0.9156 * volume_total
volume_total =
weight_total / 0.9156 =
16 / 0.9156 =
17.4749 (rounded)
volume_alcohol =
0.4 * volume_total =
6.99 ml (rounded)