Question 72854
You would need to add .2666666 quarts of pure (100%)anitfreeze to 4 quarts of a 20% solution to make it a 25% solution.
In 4 quarts, a 20% solution is .8 quarts of antifreeze.  4*(.2)=y  =>  .8=y
That can be rewritten as {{{(.8)/4 = .2}}}
To solve set up as {{{(.8+x)/(4+x)=.25}}}