Question 768499
This calculation will be simpler than if you were replacing the solution with a different concentration of alcohol solution.
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You begin with a liquid of 500 ml. volume, and you finish with a liquid of 500 ml. volume.  When you perform the replacement with the water, you are adding NO alcohol.  


Let v = volume in milliliters to remove, before replacing with the v of water.
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{{{highlight((500*(0.8)-v(0.8)+v*(0))/500=0.25)}}}
This is using decimal fractions, being my attempt to make this seem less confusing (maybe it works, maybe not).  
The equation is in this case, perfectly equivalent to
{{{highlight((500*(0.8)-v(0.8))/500=0.25)}}}
or if desired, equivalent to 
{{{highlight((0.8(500-v))/500=0.25)}}}


SOLVE FOR v.



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Do the arithmetic and find v = 343.75 ml. but in practice it should really be 343 or 344.