42. 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 solution from Cup B is poured into Cup A, then the mixture in Cup A is stirred evenly. After that, half of the alcohol solution from Cup A is poured back into Cup B. At this point, the concentration of the alcohol solution in Cup B is $\qquad$ $\%$.
Answer: 37.5