Problem Solving (PS) | Re: If a portion of a half water/half alcohol mix is replaced

Lets test 10 for total weight of solution

5 alcohol 5 water by weight (units are negligible)

x = amount we are replacing
We know that currently if we remove x units of the mix then half of x will be alcohol, the other half water.

This allows us to construct the following

(5-0.5x+0.25x)/10 = 3/10
50-2.5x=30
20=2.5x
x=8
Thus 8/10 = 0.8= 80%

---