Question 1203015
Assuming that is 20% mass per volume (?) and for the 12.5% as well


Adding some v liters of the 20% salt

{{{(0.2*20)/(v+20)=0.125}}}--------solve for v.




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The solution CAN be split into smaller parts, but look at the equation as shown.  The variable is defined.  The percents concentrations are used as their decimal equivalents.  The equation formed is to account for concentration.  One could alternatively be set up to account for amount of pure salt.


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GIVEN:  20 liters of 20% salt solution

WANTED:  12.5% salt solution

ASSIGN VARIABLE:  v, the volume of water to add, for diluting the 20 L of 20%.

ASSUMPTION:  the concentration is as weight per volume.  


Accounting for amount of pure salt before and after,
{{{0.2*20}}}, the amount of salt in both the given solution AND in the diluted solution;
{{{v+20}}}, volume in liters of the diluted solution;
The EQUATION to account for the quantity of pure salt is  {{{(0.2)(20)=(0.125)(v+20)}}}.


Next to do is just solve the equation for v.