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## IMC2020: Day 1, Problem 1
(1) \(\displaystyle w\) consists of \(\displaystyle n\) letters, all of them are from the alphabet \(\displaystyle \{\texttt{a},\texttt{b},\texttt{c},\texttt{d}\}\); (2) \(\displaystyle w\) contains an even number of letters \(\displaystyle \texttt{a}\); (3) \(\displaystyle w\) contains an even number of letters \(\displaystyle \texttt{b}\). (For example, for \(\displaystyle n=2\) there are \(\displaystyle 6\) such words: \(\displaystyle \texttt{aa}\), \(\displaystyle \texttt{bb}\), \(\displaystyle \texttt{cc}\), \(\displaystyle \texttt{dd}\), \(\displaystyle \texttt{cd}\) and \(\displaystyle \texttt{dc}\).) Armend Sh. Shabani, University of Prishtina | |||||||||||||||

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