11.2. A travel agency ran a promotion: "Buy a trip to Egypt, bring four friends who also buy a trip, and get the cost of your trip back." During the promotion, 13 customers came on their own, and the rest were brought by friends. Some of them brought exactly four new customers, while the other 100 did not bring anyone. How many tourists went to the Land of Pyramids for free?
Answer: 29.

Let each of the $x$ potential "lucky ones" bring 4 friends. Then the number of "brought" customers is $4 x$, and 13 came on their own, so the total number of tourists was $13+4 x$.

On the other hand, $x$ people brought new customers, while 100 people did not, so the total number of tourists was $x+100$.

We get the equation: $13+4 x=x+100$, from which $x=29$.

+ complete justified solution

$\pm$ the equation is correctly formulated and the correct answer is obtained, but there are no explanations

a arithmetic error is made in the solution

- the equation is incorrectly formulated
- only the answer is provided