【Analysis】According to the problem, we can assume that Class A is the class that donates the most books, $x \%$ is the percentage of the number of books it donates out of the total, and the remaining 9 classes can be divided into three groups $A$, $B$, and $C$, each consisting of 3 classes. Using the fact that the sum of the number of books donated by any 6 classes is no less than $50 \%$, we can solve for the range of $x$, and the maximum value of $x$ is the required maximum percentage.

【Solution】Solution: According to the analysis, let Class A be the class that donates the most books, $x \%$ is the percentage of the number of books it donates out of the total, and the remaining 9 classes can be divided into three groups $A$, $B$, and $C$, with the percentages of the number of books they donate being $a \%$, $b \%$, and $c \%$. Then:
$$
2(100-x)=2(a+b+c)=(a+b)+(b+c)+(c+a) \geqslant 50+50+50,
$$

Solving this, we get: $x \leqslant 25$.
$\therefore$ The maximum percentage of the number of books donated by the class that donates the most books cannot exceed $25 \%$.
When each of the 9 classes donates a percentage of the total number of books equal to $\frac{75}{9} \%=8 \frac{1}{3} \%$, then the sum of the number of books donated by any 6 of them is exactly $50 \%$ of the total, and Class A's donation is exactly $25 \%$.

Therefore, the answer is: $25 \%$.
3. (20 points) Ten graduating classes have donated books to younger students, and the total number of books donated by any six of these classes is no less than 50% of the total number of donated books. What is the maximum percentage of the total number of books that the class which donated the most could have donated? $\qquad$