<PRE><B>Question 849548</B>
1 cubic foot = {{{(576/77)}}} U.S. Gallons;------------ http://en.wikipedia.org/wiki/Cubic_feet


Volume of 5% acid in the tank:  {{{(1*pi*6^2)(ft^3)(576/77)(gallons/ft^3)=highlight_green(846*gallons)}}}


Want to add some v gallons of 20% acid to that to make 8% acid result.
{{{(846*5+20*v)/(v+846)=8}}}
{{{20v+846*5=8(v+846)}}}
{{{20v+846*5=8v+8*846}}}
{{{(20-8)v=(8-5)846}}}
{{{12v=3*846}}}
{{{v=3*846/12}}}
{{{highlight(v=211.5)}}} gallons of the 20% acid.


You can convert that volume into cubic feet, and use {{{highlight_green(y*pi*6^2=211.5(77/576))}}} and solve for y, here being the vertical increment of change for the 211.5 gallon addition of the added acid.</PRE>