7. There are three cups of solutions with equal weight, their concentrations are $10 \%, 20 \%, 45 \%$ respectively. If $\frac{1}{2}, \frac{1}{4}, \frac{1}{5}$ of the solution weights from the three cups are poured into a fourth empty cup, then the concentration of the solution in the fourth cup is $\qquad$ $\%$.
7. There are three cups of solutions with equal weight, their concentrations are 10%, 20%, 45% respectively. If $\frac{1}{2}, \frac{1}{4}, \frac{1}{5}$ of the solution weights from the three cups are poured into a fourth empty cup, then the concentration of the solution in the fourth cup is $\qquad$ $\%$.

【Solution】Solution: According to the problem:
Let the weight of the three solutions be $a$.
$$
\text { According to concentration }=\frac{\text { solute }}{\text { solution }} \times 100 \%=\frac{\frac{1}{2} \mathrm{a} \times 10 \%+\frac{1}{4} \mathrm{a} \times 20 \%+\frac{1}{5} \mathrm{a} \times 45 \%}{\frac{1}{2} \mathrm{a}+\frac{1}{4} \mathrm{a}+\frac{1}{5} \mathrm{a}} \times 100 \%=20 \%
$$

Therefore, the answer is: $20 \%$