12. (7 points) Teams 甲, 乙, and 丙 need to complete two projects, $A$ and $B$. The workload of project $B$ is $\frac{1}{4}$ more than that of project $A$. Teams 甲, 乙, and 丙 can complete project $A$ alone in 20 days, 24 days, and 30 days, respectively. To complete both projects simultaneously, Team 甲 is assigned to project $A$, and Teams 乙 and 丙 work together on project $B$; after a few days, Team 丙 is reassigned to work with Team 甲 to complete project $A$. Therefore, Team 丙 worked with Team 乙 for $\qquad$ days.
【Solution】Solution: Consider project $A$ as "1", then project $B$ is $1+\frac{1}{4}$, so the total workload: $1+1+\frac{1}{4}=\frac{9}{4}$,
work time: $\frac{9}{4} \div\left(\frac{1}{20}+\frac{1}{24}+\frac{1}{30}\right)$,
$$
\begin{array}{l}
=\frac{9}{4} \div \frac{1}{8}, \\
=18 \text { (days); }
\end{array}
$$

Answer: Teams Yi and Bing worked together for 15 days.
The answer is: 15.