A solution of 8% boric acid is to be diluted by adding a 2% boric acid solution to it. The resulting mixture is to be more than 4% but less than 6% boric acid. If we have 640 litres of the 8% solution, how many litres of the 2% solution will have to be added?

Explanation

To solve the problem step by step, let's define the variables and set up the inequalities based on the information given.

### Step 1: Define Variables
Let:
- \( x \) = the volume of the 2% boric acid solution to be added (in liters).
- The volume of the 8% boric acid solution is given as 640 liters.

### Step 2: Set Up the Inequalities
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