13. There are two identical cups, A and B. Cup A contains half a cup of clear water, and Cup B is full of a $50 \%$ alcohol solution. First, half of the alcohol solution from Cup B is poured into Cup A, then the mixture in Cup A is stirred evenly, and half of the alcohol solution from Cup A is poured back into Cup B. What fraction of the solution in Cup B is alcohol at this point?
13. The alcohol in cup B is $\frac{3}{8}$ of the solution.
13.【Solution】After half of the alcohol solution in cup B is poured into cup A for the first time, the solution in cup A contains $25 \%$ alcohol; the second time, when the alcohol solution in cup A is poured into cup B, the alcohol in cup B is $\frac{1}{2} \times 25 \% + \frac{1}{2} \times 50 \% = \frac{1}{8} + \frac{1}{4} = \frac{3}{8}$.