11.1. While walking in the park, Seryozha and Misha stumbled upon a meadow surrounded by lindens. Seryozha walked around the meadow, counting the trees. Misha did the same but started from a different tree (although he went in the same direction). The tree that was the $20-\mathrm{th}$ for Seryozha was the $7-\mathrm{th}$ for Misha, and the tree that was the $7-\mathrm{th}$ for Seryozha was the 94-th for Misha. How many trees grew around the meadow?
# Answer: 100.

Solution. First method. Let there be $n$ trees growing around the glade. We will calculate in two ways the number of intervals between the two trees mentioned in the problem's condition. During Sergei's walk: $20-7=13$. During Misha's walk: $7+(n-94)=n-87$. Therefore, $n-87=13$, which means $n=100$.

Second method. Between the first and second mentioned trees, Misha counted $94-7-1=86$ more trees. And Sergei counted $20-7-1=12$ trees between the second tree and the first tree. Thus, around the glade, there are $86+12=98$ trees plus the two mentioned trees, which makes exactly 100 trees.

## Grading criteria:

+ the correct answer and a fully justified solution are provided

partially correct reasoning is provided, but there is an error of one tree in the "jump"

- only the answer is provided