Question 460415
let x = # liters of pure acid to be added.  Pure acid is 100% acid (This sounds stupid, but it really is important.)  
Then 

1.00x + 0.10*80 = 0.30(x+80)

==> x + 8 = 0.30x + 24

==> 0.70x = 16

==>  {{{x = 22&6/7}}} liters.