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 there are `640` litres of the `8%` solution, how many litres of `2%` solution will have to be added?

Explanation

To solve the problem step by step, we will set up the inequalities based on the information provided and solve for the variable \( x \), which represents the volume of the 2% boric acid solution to be added.

### Step 1: Define the Variables
Let \( x \) be the number of liters of the 2% boric acid solution to be added.

### Step 2: Calculate the Amount of Boric Acid in Each Solution
- The amount of boric acid in the 8% solution:
\[
...