9. From two groups of skiers with a total of 100 people, a team of 15 people was formed. The first group contributed $p \%$ of its members, and the second group contributed $-10 \%$ of its members. How many skiers are there in each group?

Answer: 50 and 50, or 20 and 80, or 10 and 90.

Solution: Let $x$ and $y$ be the number of skiers in these groups. Then

$$
\left\{\begin{array}{l}
x+y=100 \\
p x+10 y=1500
\end{array}\right.
$$

Therefore, $x=\frac{500}{p-10}$ and $y=\frac{100(p-15)}{p-10}$, where $15<p<100$. Since $x$ is an integer, the number $p-10$ must be a divisor of 500. By enumeration, we find the answer:

| $p$ | $x$ | $y$ |
| :---: | :---: | :---: |
| 20 | 50 | 50 |
| 35 | 20 | 80 |
| 60 | 10 | 90 |

B. Solve problems 10-12 in as many ways as possible. Different methods are considered to be those that use different mathematical ideas, as well as different technical approaches to implementing the same idea. Indicate the place of each of the methods used in the school mathematics curriculum.
5.I.9. Comment. The error in the given solution is that, by the meaning of the problem, the numbers $x$ and $p x$ are natural, while the number $p$ can be rational. Therefore, the problem has more solutions than those provided in its solution.