3. (10 points) A barrel of oil, the oil it contains is $\frac{3}{5}$ of the barrel's full capacity. After selling 18 kilograms, 60% of the original oil remains. Therefore, this barrel can hold $\qquad$ kilograms of oil.
【Analysis】First, consider the original mass of the oil as a unit "1". After selling 18 kilograms, 60% of the original oil remains. Therefore, the mass sold is (1-60%) of the original oil mass, which corresponds to 18 kilograms. According to the meaning of fraction division, dividing 18 kilograms by (1-60%) will give the original mass of the oil. This is also $\frac{3}{5}$ of the total capacity of the oil drum. Then, considering the total capacity of the oil drum as a unit "1", and using the meaning of fraction division again, dividing the original mass of the oil by $\frac{3}{5}$ will give the total capacity of the oil drum in kilograms.

【Solution】Solution: $18 \div(1-60 \%)$
$$
\begin{array}{l}
=18 \div 40 \% \\
=45 \text { (kg) } \\
45 \div \frac{3}{5}=75 \text { (kg) }
\end{array}
$$

Answer: This oil drum can hold 75 kilograms of oil.
Therefore, the answer is: 75.