Question 712355
Candy starts with 9 pounds mixture * 0.20 fraction chocolate = 1.8 pounds of chocolate.  She will remove x pounds of this 0.20 fraction chocolate, add x pounds of pure chocolate, still have 9 pounds of mixture, and this now be 50% chocolate.



{{{(9*0.20-0.20x+1*x)/9=0.50}}}, check this carefully to know how the numbers work.  I'm using decimal values instead of percents.


{{{0.2*9+0.8*x=9*0.50}}}
{{{x=9(0.5-0.2)/0.8}}}
{{{x=9(3/8)}}}
{{{x=27/8}}}
{{{x=3&3/8}}} pounds


That value is worth checking.  It does work.  Yes I did check.