Question

How much pure acid should be mixed with 2 gallons of 80% acid solution in order to get 90% acid​ solution?

Answer 1

Add 2 gallons of pure acid to 2 gallons of 80% acid to make a 90% acid solution.

Check answer:
8*2 = 1.6 gal of pure acid in the original 2 gal.
1.6+ 2 = 3.6 gal of pure acid in the final solution.
3.6/4 * 100 = 90%