Question 1008876
v, the amount of alcohol to be removed and replaced with v volume of water.  Note, the water
does not contribute any quantity of alcohol, being 0% alcohol.  The volume of the solution
after evaporation and addition will still be the same 3 liters.


Using fractions instead of percents,
{{{(0.4*3-v)/3=0.3}}}
and understand the starting amount of alcohol was  0.4*3 LITERS, and v liters of alcohol
to remove is as 100% alcohol; the starting solution is 40% alcohol.


Now, just solve for v.