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 need to determine how many liters of a 30% acid solution must be added to 600 liters of a 12% acid solution to achieve a final mixture that has an acid concentration between 15% and 18%.

### Step 1: Define the Variables
Let \( x \) be the number of liters of the 30% acid solution that we need to add.

### Step 2: Calculate the Acid Content in Each Solution
- The amount of acid in the 600 liters of 12% solution:
\[
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