A solution has same concentration everywhere. So, 12 litres of solution will have same concentration of milk and water as the entire solution.

Let the capacity of drum be $x$ litres.

Amount of milk $ = .18x$

Amount of milk reduced $ = 0.18 \times 12 = 2.16$ litres which amounts to $18 - 15 = 3\%$ of $x = 0.03x$.

So, $x = \frac{2.16}{0.03} = 72$ litres.