Question 900947
Two gallons of 40% acid,
You must include the density of the acid solutions.  IFF the concentrations are as volume per volume, then the density is not really necessary; but then the answer to the question must be in GALLONS of the pure acid.  


ONLY if you have all concentrations as volume per volume, then:
Let Q be the VOLUME of pure acid to add.
{{{(2*40+Q*100)/(Q+2)=70}}}