#### Answer

There are $90$ liters of pure acid in the 120-liter solution.

#### Work Step by Step

Let
$x$ = number of liters of solution
$y$ = amount of pure acid in the solution
Note that $75\%=0.75$.
Thus, the amount of pure acid in the solution is given by the equation:
$y=0.75x$
Therefore, in a 120-liter solution, the amount of pure acid is:
$y=0.75x
\\y=0.75(120)$
$\\y=90$ liters