Question 847856
The concentration of alcohol in the 100 ml. alcohol solution is still unknown.  You do not then have a way to find how much water to add to make 20% alcohol.  


Assuming percents are vol/vol, and you have 100 ml. of some  c percent alcohol, and you want to add 40 ml. of water; AND also some v amount of 100% alcohol to make 20% solution, ...


Amount of pure alcohol = {{{100*c+100*v}}}
Amount of solution result = {{{100+40+v}}}
Equation for adjustments to be 20% alcohol:  {{{(100c+100v)/(100+40+v)=20}}};


Simplified:  {{{(100(c+v))/(140+v)=20}}}
further simplifiable based on the convenient numbers,
{{{(10(c+v))/(140+v)=2}}}
OR
{{{highlight(5(c+v)/(140+v)=1)}}}----simplified equation to solve for v or c.


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