Question 662482
You can add some water, or solvent in some cases, same thing.
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In a 50% solution, you have 50% "stuff", I'll say it is antifreeze.
The other 50% I will say is water
Let {{{ x }}} = the total amount of solution you start with, in
gallons, or whatever
{{{ .5x }}} = amount of antifreeze
Add {{{ w }}} amount of water
Now you have {{{ x + w }}} amount of solution 
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I can say
{{{ .5x / ( x + w ) = .25 }}}
{{{ .5x = .25*( x + w ) }}}
{{{ .5x = .25x + .25w }}}
{{{ .25w = .25x }}}
{{{ w = x }}}
So, this says you add an amount of water equal to
the amount of solution you started with.
If you started with 1 gallon of solution, now you
have 2 gallons of solution and .5 gallons of antifreeze
.5 / 2 = 1/4 or 25%