What quantity of a 70% alcohol solution must be mixed with a 0% alcohol solution to produce 1000ml of an 18% solution
1,000 ml of 18% solution will contain 180 ml alcohol.
Let V1 be the volume of 70% alcohol solution used.
Let V2 be the volume of 0% alcohol solution used.
V1 @ 70% = 0.7V1 ml of alcohol
V2 @ 0% = 0 ml of alcohol
Summing these alcoholvolumes, 0.7V1 + 0 = 0.7V1 ml
i.e. 0.7V1 = 180
V1 = 257.14ml of 70% alcohol
V2 = 1,000 -257.14 = 742.86 ml
The final solution of 1,000 ml of 18% alcohol solution is made up from
257.14 ml of 70% solution with 742.86 ml of 0% solution