6. A container is filled with an alcohol solution. The first time, $10 \mathrm{~L}$ is poured out and then water is added to fill it up; the second time, $6 \mathrm{~L}$ is poured out and then water is added to fill it up again. At this point, the volume ratio of alcohol to water in the container is 7: 13. Then the capacity of this container is ( $) \mathrm{L}$.
(A) 18
(B) 20
(C) 24
(D) 30
6. B.

Let the volume of the container be $x \mathrm{~L}$. Then
$$
\frac{x-10-\frac{x-10}{x} \times 6}{x}=\frac{7}{20} \text {. }
$$

Solving, we get $x_{1}=20, x_{2}=\frac{60}{13}$ (rejected).