Question 606225
Pure acid-- this means that we have 100% acid 

Using that fact:
we have the following equation:
{{{x*1.00+5*0.4=(x+5)*0.50}}}

where x is the amount of pure acid that is added (also note that we will end up with x+5 gallons in the end)

From here it is a matter of algebra and solving for x.  

try it yourself from this step, and let me know if you have any difficulties:)