Question 847174
Drain v quarts of the 25% and replace with v quarts of the pure, 100%.


Amount of antifreeze will be:  {{{5*25-v*25+v*100}}}, which is 100 times larger because of using the percents unit.  Simplify partway into {{{25*5+(100-25)v}}}.


The equation for wanting 50% antifreeze result is:
{{{highlight((25*5+(100-25)v)/5=50)}}}.
Solve for v.


A next step could be the combined step,  {{{125+75v=250}}}....
{{{75v=125}}}
{{{v=125/75=(5*5*5)/(3*5*5)}}}
{{{highlight(v=5/3=1&2/3)}}} quarts