A cosmetic manufacturer initially prepared 300 liters of a shampoo base containing 25% essential oil. After using 60 liters of this base, he mixed in equal volumes of pure essential oil and distilled water. As a result, the ratio of distilled water to essential oil changed to 5:2. How many liters of essential oil were added to the mixture?

The correct answer is 20 liters.

Step 1: Calculate the initial amounts of essential oil and distilled water.
The manufacturer initially prepares 300 liters of shampoo base containing 25% essential oil.
Initial volume of essential oil = 25% of 300 liters = 75 liters.
Initial volume of distilled water = 300 - 75 = 225 liters.

Step 2: Calculate the remaining amounts after 60 liters of base are used.
Remaining volume of base = 300 - 60 = 240 liters.
Since the concentration remains 25%, the remaining base contains:
Essential oil = 25% of 240 liters = 60 liters.
Distilled water = 75% of 240 liters = 180 liters.

Step 3: Formulate the equation after adding equal volumes.
Let $x$ be the volume (in liters) of essential oil added. Since equal volumes of pure essential oil and distilled water are added, $x$ liters of distilled water are also added.
New volume of essential oil = $60+x$ liters.
New volume of distilled water = $180+x$ liters.

The ratio of distilled water to essential oil becomes 5:2:
$\frac{180+x}{60+x}=\frac{5}{2}$

Step 4: Solve for $x$.
Cross-multiplying yields:
$2(180+x)=5(60+x)$
$360+2x=300+5x$
$360-300=5x-2x$
$60=3x$
$x=20$

Therefore, 20 liters of essential oil were added to the mixture.