點dilute 30% alcohol到 20% & 10%

always remember the rule:
c1 x V1 = c2 x V2
that means, product of concentration and volume before dilution will be equal to that after dilution.

for example, the volume of 30% alcohol = V1, then
30% x V1 = 20% x V2
V2 = V1 x (3/2)
meaning final volume of dilution will be 3/2 of original volume.
the difference in volume, or amount of water added = V2-V1 = 0.5V1 .

and then, from 20% to 10%...
20% x V2 = 10% x V3
V3 = 2 x V2 .
meaning that final volume should be two times of volume of 20% alcohol.
the difference in volume, or amount of water added to dilute 20% alcohol
= V3-V2 = V2 .