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, we need to determine how many liters of a 2% boric acid solution should be added to 640 liters of an 8% boric acid solution so that the resulting mixture has a boric acid concentration between 4% and 6%.

### Step-by-Step Solution:

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

2. **Calculate the Amount of Boric Acid in Each Solution**:
...