7. Pairings (from 8th grade. 2 points). At the tournament, 10 tennis players arrived, 4 of whom are from Russia. According to the rules, for the first round, the players are randomly paired. Find the probability that in the first round all Russian players will be paired only with other Russian players.
Solution. Let's take one Russian woman. In pa-

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the probability that another Russian woman will be paired with her is $\frac{3}{9}=\frac{1}{3}$.

Given this condition, let's consider the two remaining Russian women. Reasoning similarly, we get that they will be paired together with a probability of $\frac{1}{7}$. Therefore, the desired probability is $\frac{1}{3} \cdot \frac{1}{7}=\frac{1}{21}$.

Answer: $\frac{1}{21}$ or approximately 0.048.