A manufacturer has 600 litres of a 12% solution of acid. How many litres of a 30% acid solution must be added to it so that acid content in the resulting mixture will be more than 15% but less than 18%?

Explanation

To solve the problem step by step, we will define the variables, set up the inequalities based on the given conditions, and solve them accordingly.

### Step 1: Define the variables
Let \( x \) be the number of liters of the 30% acid solution that needs to be added.

### Step 2: Calculate the amount of acid in the existing solution
The existing solution is 600 liters of a 12% acid solution. The amount of acid in this solution can be calculated as:
\[
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