Let \( x \) be the number of milliliters of the substance containing 6% sandalwood. The chemists combined 5 milliliters of the substance containing 2% sandalwood with \( x \) milliliters of the substance containing 6% sandalwood.
The total volume of the new mixture is:
\[ 5 + x \]
Next, we need to calculate the amount of sandalwood in each substance:
- The amount of sandalwood from the 5 milliliters of the 2% substance is:
\[ 0.02 \times 5 = 0.1 \text{ milliliters} \]
- The amount of sandalwood from \( x \) milliliters of the 6% substance is:
\[ 0.06x \text{ milliliters} \]
The total amount of sandalwood in the final mixture, which is 5% of the total volume, can be expressed as:
\[ 0.05(5 + x) \]
Now we can set up the equation for the total amount of sandalwood in the mixture:
\[ 0.1 + 0.06x = 0.05(5 + x) \]
Next, we'll simplify the right side of the equation:
\[ 0.1 + 0.06x = 0.25 + 0.05x \]
Now we'll rearrange the equation to isolate \( x \):
\[ 0.06x - 0.05x = 0.25 - 0.1 \]
This simplifies to:
\[ 0.01x = 0.15 \]
Now, dividing both sides by 0.01 gives:
\[ x = \frac{0.15}{0.01} = 15 \]
Thus, the chemists must have used 15 milliliters of the substance containing 6% sandalwood.