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