7.1. Ten guides were leading ten tour groups through the halls of the Hermitage. The groups did not necessarily have equal numbers, but on average, there were 9 people in each group. When one of the groups finished their tour, the average number of tourists per group decreased to 8 people. How many people were in the group that finished the tour?
Answer: 18.

Solution: The total number of excursionists initially was $9 \cdot 10=90$. After one of the groups finished the excursion, $8 \cdot 9=72$ excursionists remained. Therefore, the group that finished the excursion had $90-72=18$ excursionists.

Comment: A correct answer without justification - 0 points.