Geodesic
Tři snadné (?) úlohy
Rozhledy matematicko-fyzikální, Tome 93 (2018) no. 1, pp. 39-40 Cet article a éte moissonné depuis la source Czech Digital Mathematics Library

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The article presents three (not so) easy mathematical problems. In the first problem, the task is to find such a natural number $n$ that the product $999\cdot n$ contains only the digits 1. The second problem and the third problem are based on the parity of natural numbers.
The article presents three (not so) easy mathematical problems. In the first problem, the task is to find such a natural number $n$ that the product $999\cdot n$ contains only the digits 1. The second problem and the third problem are based on the parity of natural numbers. 
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     author = {Calda, Emil},
     title = {T\v{r}i snadn\'e (?) \'ulohy},
     journal = {Rozhledy matematicko-fyzik\'aln{\'\i}},
     pages = {39--40},
     year = {2018},
     volume = {93},
     number = {1},
     language = {cs},
     url = {http://geodesic.mathdoc.fr/item/RMF_2018_93_1_a5/}
}
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Calda, Emil. Tři snadné (?) úlohy. Rozhledy matematicko-fyzikální, Tome 93 (2018) no. 1, pp. 39-40. http://geodesic.mathdoc.fr/item/RMF_2018_93_1_a5/

[1] Calda, E.: Sbírka řešených úloh – Středoškolská matematika pod mikroskopem, 1. kapitola. Prometheus, Praha, 2006.