Question 776256
What is the concentration of the Hydrochloric Acid to add?  That datum is necessary.  Otherwise, we have an extra variable with no given value.


Let h = concentration of your concentrated hydrochloric acid
L = 30% HCl
T = 40% HCl
U = 12 gallons of the 30%
v = volume gallons of the concentrated hydrochloric acid HCl still of unknown value


You do not know nor need the expected amount of volume of the 40% target concentration, or if so, it is not part of this problem.


Equation for the mixing is this:
{{{(LU+hv)/(U+v)=T}}}
You can solve the equation for v but you cannot get a value for it until you have a value for h.