Question 783242
The capacity of the full container is not given.  Call it C.  The volume, v units of solution would be removed and replaced with v units of the 100% pure material to increase the percentage concentration.  The volume of mixture before and after the change is the same.


{{{highlight((C*30-v*30+v*100)/C=50)}}}
You can solve for v in terms of C.  


{{{C*30+(100-30)v=50C}}}
{{{70v=50C-30C}}}
{{{70v=20C}}}
{{{7v=2C}}}
{{{highlight(v=(2/7)C)}}}