20. (6 points) As shown in the figure, the upper base $AD$ of trapezoid $ABCD$ is 12 cm long, the height $BD$ is 18 cm long, and $BE=2DE$. Then the length of the lower base $BC$ is
cm.
24
【Solution】Since $A D / / B C$,
thus $\triangle A D E$ and $\triangle C B E$ are similar, because $B E=2 D E$, that is, $D E: B E=1: 2$, so $A D: C B=1: 2$, and since $A D=12$ cm, then $B C=12 \times 2=24$ (cm), answer: the length of the lower base $B C$ is 24 cm. Therefore, the answer is: 24.