Question 710520
A lemonade that is 25% lemon juice would have 
{{{25/100=0.25}}} gallons of lemon juice in
exactly {{{1}}} gallon of lemonade.
A lemonade that is 20% lemon juice has
{{{20/100=0.20}}} gallons of lemon juice in
exactly {{{1}}} gallon of lemonade.
If you add {{{5/100}}} gallon lemon juice, you will have a total of
{{{1&5/100}}} gallons = {{{1.05}}} gallons of lemonade containing {{{20/100+5/100=25/100}}} or {{{0.20+0.05=0.25}}} gallons of lemon juice.
That will not be exactly 25% lemon juice.
In fact, the percentage is {{{(0.25/1.05)*100}}}% = about {{{23.8}}}%

{{{x}}} = gallons of lemon juice to add
{{{1+x}}} = final volume of lemonade (in gallons)
{{{1*0.20}}} = gallons of lemon juice in original {{{1}}} gallon of lemonade
{{{(1+x)*0.25}}} = gallons of lemon juice in final mixture
{{{1*0.20+x=(1+x)*0.25}}}
{{{0.20+x=0.25+0.25x}}}
{{{0.20+x-0.25x=0.25}}}
{{{x-0.25x=0.25-0.20}}}
{{{0.75x=0.05}}} --> {{{0.75x/0.75=0.05/0.75}}} --> {{{highlight(x=1/15)}}} = approx. {{{0.0633}}} gallons.
The number does not get prettier in quarts:
{{{1/15}}}gallon = {{{4/15}}}quarts