Alexander B. answered • 09/15/15

PhD in Engineering with 15+ yrs of Math Teaching Experience

Let's convert percentage into decimals and assume the amount of the solution poured off and replaced with pure acid is

**x**. The following linear equation provides the solution for

**x**:

(30-x)*0.4 + x = 30*0.7

in a simplified form

0.6x + 12 = 21

This linear equation can be solved either manually or by using online Linear Equation Solver (http://examn8.com/LinearEquations.aspx)

The answer is:

**x=15 ml**

Hope this may help.